UPGRADING

PHP 8.1 UPGRADE NOTES

1. Backward Incompatible Changes
2. New Features
3. Changes in SAPI modules
4. Deprecated Functionality
5. Changed Functions
6. New Functions
7. New Classes and Interfaces
8. Removed Extensions and SAPIs
9. Other Changes to Extensions
10. New Global Constants
11. Changes to INI File Handling
12. Windows Support
13. Other Changes
14. Performance Improvements

========================================
1. Backward Incompatible Changes
========================================

- Core:
  . Access to the $GLOBALS array is now subject to a number of restrictions.
    Read and write access to individual array elements like $GLOBALS['var']
    continues to work as-is. Read-only access to the entire $GLOBALS array also
    continues to be supported. However, write access to the entire $GLOBALS
    array is no longer supported. For example, array_pop($GLOBALS) will result
    in an error.
    RFC: https://wiki.php.net/rfc/restrict_globals_usage
  . Passing null to a non-nullable argument of a built-in function is
    deprecated. This matches the behavior of user-defined functions, where null
    is never accepted by non-nullable arguments.
    user-defined functions.

        var_dump(str_contains("foobar", null));
        // Deprecated: Passing null to parameter #2 ($needle) of type string
        //             is deprecated

    RFC: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg
  . When a method using static variables is inherited (but not overridden), the
    inherited method will now share static variables with the parent method.

        class A {
            public static function counter() {
                static $counter = 0;
                $counter++;
                return $counter;
            }
        }
        class B extends A {}

        var_dump(A::counter()); // int(1)
        var_dump(A::counter()); // int(2)
        var_dump(B::counter()); // int(3), previously int(1)
        var_dump(B::counter()); // int(4), previously int(2)

    This means that static variables in methods now behave the same way as
    static properties.
    RFC: https://wiki.php.net/rfc/static_variable_inheritance
  . Most non-final internal methods now require overriding methods to declare a
    compatible return type, otherwise a deprecated notice is emitted during
    inheritance validation.
    In case the return type cannot be declared for an overriding method due to
    PHP cross-version compatibility concerns, a `#[ReturnTypeWillChange]`
    attribute can be added to silence the deprecation notice.
    RFC: https://wiki.php.net/rfc/internal_method_return_types

- Fileinfo:
  . The fileinfo functions now accept and return, respectively, finfo objects
    instead of resources. Return value checks using is_resource()
    should be replaced with checks for `false`.

- FTP:
  . The FTP functions now accept and return, respectively, FTP\Connection objects
    instead of resources. Return value checks using is_resource()
    should be replaced with checks for `false`.

- IMAP:
  . The IMAP functions now accept and return, respectively, IMAP\Connection objects
    instead of resources. Return value checks using is_resource()
    should be replaced with checks for `false`.

- LDAP:
  . The LDAP functions now accept and return, respectively, LDAP\Connection objects
    instead of "ldap link" resources. Return value checks using is_resource()
    should be replaced with checks for `false`.
  . The LDAP functions now accept and return, respectively, LDAP\Result objects
    instead of "ldap result" resources. Return value checks using is_resource()
    should be replaced with checks for `false`.
  . The LDAP functions now accept and return, respectively, LDAP\ResultEntry
    objects instead of "ldap result entry" resources. Return value checks using
    is_resource() should be replaced with checks for `false`.

- MySQLi:
  . mysqli_fetch_fields() and mysqli_fetch_field_direct() will now always return
    zero for max_length. You can compute this information by iterating over the
    result set and taking the maximum length. This is what PHP was doing
    internally previously.
  . The MYSQLI_STMT_ATTR_UPDATE_MAX_LENGTH option no longer has an effect.
  . The MYSQLI_STORE_RESULT_COPY_DATA option no longer has an effect.
  . The default error handling mode has been changed from "silent" to
    "exceptions". See https://www.php.net/manual/en/mysqli-driver.report-mode.php
    for details of behavior changes and how to explicitly set this attribute. To
    keep the old behavior, use mysqli_report(MYSQLI_REPORT_OFF);
    RFC: https://wiki.php.net/rfc/mysqli_default_errmode
  . Classes extending mysqli_stmt::execute() will be required to specify the
    additional parameter now.
    RFC: https://wiki.php.net/rfc/mysqli_bind_in_execute
  . mysqli::connect() will now return true instead of null on success.

- MySQLnd:
  . The mysqlnd.fetch_data_copy ini setting has been removed. However, this
    should not result in user-visible behavior changes.

- OpenSSL:
  . EC private keys will now be exported in PKCS#8 format rather than
    traditional format, just like all other keys.
  . openssl_pkcs7_encrypt() and openssl_cms_encrypt() will now default to
    using AES-128-CBC rather than RC2-40. The RC2-40 cipher is considered
    insecure and not enabled by default in OpenSSL 3.

- PDO:
  . PDO::ATTR_STRINGIFY_FETCHES now also stringifies values of type bool to
    "0" or "1". Previously booleans were not stringified.
  . Calling bindColumn() with PDO::PARAM_LOB (and assuming stringification is
    not enabled) will now consistently bind a stream result, as documented.
    Previously the result would be either a stream or a string depending on the
    used database driver and the time the binding is performed.

- PDO MySQL:
  . Integers and floats in result sets will now be returned using native PHP
    types instead of strings when using emulated prepared statements. This
    matches the behavior of native prepared statements. You can restore the
    previous behavior by enabling the PDO::ATTR_STRINGIFY_FETCHES option.

- PDO SQLite:
  . Integers and floats in results sets will now be returned using native PHP
    types. You can restore the previous behavior by enabling the
    PDO::ATTR_STRINGIFY_FETCHES option.

- PgSQL:
  . The PgSQL functions now accept and return, respectively, \PgSql\Connection
    objects instead of "pgsql link" resources. Return value checks using
    is_resource() should be replaced with checks for `false`.
  . The PgSQL functions now accept and return, respectively, \PgSql\Result
    objects instead of "pgsql result" resources. Return value checks using
    is_resource() should be replaced with checks for `false`.
  . The PgSQL functions now accept and return, respectively, \PgSql\Lob
    objects instead of "pgsql large object" resources. Return value checks
    using is_resource() should be replaced with checks for `false`.

- Phar:
  . To comply with the ArrayAccess interface, Phar::offsetUnset() and
    PharData::offsetUnset() no longer return a boolean.

- PSpell:
  . The PSpell functions now accept and return, respectively, PSpell\Dictionary objects
    instead of "pspell" resources. Return value checks using is_resource()
    should be replaced with checks for `false`.
  . The PSpell functions now accept and return, respectively, PSpell\Config objects
    instead of "pspell config" resources. Return value checks using is_resource()
    should be replaced with checks for `false`.

- Standard:
  . version_compare() no longer accepts undocumented operator abbreviations.
  . htmlspecialchars(), htmlentities(), htmlspecialchars_decode(),
    html_entity_decode() and get_html_translation_table() now use
    ENT_QUOTES | ENT_SUBSTITUTE rather than ENT_COMPAT by default. This means
    that ' is escaped to ' while previously it was left alone.
    Additionally, malformed UTF-8 will be replaced by a Unicode substitution
    character, instead of resulting in an empty string.
  . debug_zval_dump() will now print reference wrappers with their refcount,
    instead of only prepending a "&" to the value. This more accurately models
    reference representation since PHP 7.0.
  . debug_zval_dump() will now print "interned" instead of a dummy refcount of
    one for interned strings and immutable arrays.

- SPL:
  . SplFixedArray will now be JSON encoded like an array.

========================================
2. New Features
========================================

- Core:
  . It is now possible to specify octal integer by using the explicit "0o"/"0O"
    prefix similar to hexadecimal ("0x"/"0X) and binary ("0b"/"0B") integer
    literals.
    RFC: https://wiki.php.net/rfc/explicit_octal_notation
  . Added support for array unpacking with strings keys.
    RFC: https://wiki.php.net/rfc/array_unpacking_string_keys
  . Added support for enumerations.
    RFC: https://wiki.php.net/rfc/enumerations
  . Added support for never return type
    RFC: https://wiki.php.net/rfc/noreturn_type
  . Added support for fibers.
    RFC: https://wiki.php.net/rfc/fibers
  . It is now possible to use "new ClassName()" expressions as parameter
    default values, static variable and global constant initializers, as well
    as attribute arguments. Objects can also be passed to `define()` now.
    RFC: https://wiki.php.net/rfc/new_in_initializers
  . Closures for callables can now be created using the syntax `myFunc(...)`,
    which is the same as `Closure::fromCallable('myFunc')`. Yes, the `...` is
    part of the syntax, not an omission.
    RFC: https://wiki.php.net/rfc/first_class_callable_syntax
  . File uploads now provide an additional full_path key, which contains the
    full path (rather than just the basename) of the uploaded file. This is
    intended for use in conjunction with "upload webkitdirectory".
  . It is now allowed to specify named arguments after an argument unpack, e.g.
    foo(...$args, named: $arg).
  . Added support for intersection types.
    They cannot be combined with union types.
    RFC: https://wiki.php.net/rfc/pure-intersection-types
  . Added support for the final modifier for class constants.
    RFC: https://wiki.php.net/rfc/final_class_const
  . Added support for readonly properties.
    RFC: https://wiki.php.net/rfc/readonly_properties_v2

- Curl:
  . Added CURLOPT_DOH_URL option.
  . Added certificate blob options when for libcurl >= 7.71.0:

        CURLOPT_ISSUERCERT_BLOB
        CURLOPT_PROXY_ISSUERCERT
        CURLOPT_PROXY_ISSUERCERT_BLOB
        CURLOPT_PROXY_SSLCERT_BLOB
        CURLOPT_PROXY_SSLKEY_BLOB
        CURLOPT_SSLCERT_BLOB
        CURLOPT_SSLKEY_BLOB

  . Added CURLStringFile, which can be used to post a file from a string rather
    than a file:

        $file = new CURLStringFile($data, 'filename.txt', 'text/plain');
        curl_setopt($curl, CURLOPT_POSTFIELDS, ['file' => $file]);

- FPM:
  . Added openmetrics status format. It can be used by Prometheus to fetch FPM
    metrics.
  . Added new pool option for the dynamic process manager called
    pm.max_spawn_rate. It allows to start number of children in a faster rate
    when dynamic pm is selected. The default value is 32 which was the previous
    hard coded value.

- GD:
  . Avif support is now available through the imagecreatefromavif() and
    imageavif() functions, if libgd has been built with avif support.

- hash:
  . The following functions have changed signatures, to support an optional `$options` argument:

    - function hash(string $algo, string $data, bool $binary = false, array $options = []): string|false {}
    - function hash_file(string $algo, string $filename, bool $binary = false, array $options = []): string|false {}
    - function hash_init(string $algo, int $flags = 0, string $key = "", array $options = []): HashContext {}

    The additional `$options` argument can be used to pass algorithm specific data.

  . Added MurmurHash3 with streaming support. The following variants are implemented:

    - murmur3a, 32-bit hash
    - murmur3c, 128-bit hash for x86
    - murmur3f, 128-bit hash for x64

    The initial hash state can be passed through the `seed` key in the `$options` array, for example:

    ```php
    $h = hash("murmur3f", $data, options: ["seed" => 42]);
    echo $h, "\n";
    ```

    A valid seed value is within the range from 0 to the platform defined UINT_MAX, usually 4294967295.

  . Added xxHash. The implementation brings in the following arguments

    - xxh32, 32-bit hash
    - xxh64, 64-bit hash
    - xxh3, 64-bit hash
    - xxh128, 128-bit hash

    The initial hash state can be passed through the `seed` key in the `$options` array, for example:

    ```php
    $h = hash("xxh3", $data, options: ["seed" => 42]);
    echo $h, "\n";
    ```

    Secret usage is supported through passing the `secret` key in the `$options` array, too:

    ```php
    $h = hash("xxh3", $data, options: ["secret" => "at least 136 bytes long secret here"]);
    echo $h, "\n";
    ```

    Note, that the quality of the custom secret is crucial for the quality of the resulting hash. It is
    highly recommended for the secret to use the best possible entropy.

- MySQLi:
  . The mysqli.local_infile_directory ini setting has been added, which can be
    used to specify a directory from which files are allowed to be loaded. It
    is only meaningful if mysqli.allow_local_infile is not enabled, as all
    directories are allowed in that case.
  . Binding in execute has been added to mysqli prepared statements.
    Parameters can now be passed to mysqli_stmt::execute as an array.
    RFC: https://wiki.php.net/rfc/mysqli_bind_in_execute
  . A new method has been added to mysqli_result called mysqli_fetch_column().
    It allows for fetching single scalar values from the result set.
    RFC: https://wiki.php.net/rfc/mysqli_fetch_column

- PDO MySQL:
  . The PDO::MYSQL_ATTR_LOCAL_INFILE_DIRECTORY attribute has been added, which
    can be used to specify a directory from which files are allowed to be
    loaded. It is only meaningful if PDO::MYSQL_ATTR_LOCAL_INFILE is not
    enabled, as all directories are allowed in that case.

- PDO SQLite:
  . SQLite's "file:" DSN syntax is now supported, which allows specifying
    additional flags. This feature is not available if open_basedir is set.
    Example:

        new PDO('sqlite:file:path/to/sqlite.db?mode=ro')

- Posix:
  . Added POSIX_RLIMIT_KQUEUES and POSIX_RLIMIT_NPTS. These rlimits are only
    available on FreeBSD.

- Standard:
  . fputcsv() now accepts a new "eol" argument which allow to define a custom
    eol sequence, the default remains the same and is "\n".

- SPL:
  . SplFileObject::fputcsv() now accepts a new "eol" argument which allow to
    define a custom eol sequence, the default remains the same and is "\n".

========================================
3. Changes in SAPI modules
========================================

- CLI:
  . Using -a without the readline extension will now result in an error.
    Previously, -a without readline had the same behavior as calling php without
    any arguments, apart from printing an additional "Interactive mode enabled"
    message. This mode was not, in fact, interactive.

- phpdbg:
  . Remote functionality from phpdbg has been removed.

========================================
4. Deprecated Functionality
========================================

- Core:
  . Implementing the Serializable interface without also implementing
    __serialize() and __unserialize() has been deprecated. You should either
    implement the new methods (if you only support PHP 7.4 and higher) or
    implement both (if you support older PHP versions as well).
    RFC: https://wiki.php.net/rfc/phase_out_serializable
  . Implicit conversion of floats to integers that result in loss of precision,
    e.g. a truncation from 1.9 to 1, is deprecated. This affects array keys,
    int parameter and return types, and operators working on integers.
    RFC: https://wiki.php.net/rfc/implicit-float-int-deprecate
  . Calling a static method or accessing a static property directly on a trait
    is deprecated. Static methods and properties should only be accessed on a
    class using the trait.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . Returning a non-array from __sleep will raise a warning
  . Returning by reference from a void function is deprecated.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . Automatic conversion of "false" into an empty array on write operands is
    deprecated.
    RFC: https://wiki.php.net/rfc/autovivification_false

- Ctype:
  . Passing a non-string value to ctype_*() functions is deprecated. A future
    version of PHP will make ctype_*() accept a string argument, which means
    that either only strings will be accepted (strict_types=1) or inputs may be
    converted to string (strict_types=0). In particular, using ctype_*($cp) to
    check whether an ASCII codepoint given as integer satisfies a given ctype
    predicate will no longer be supported. Instead ctype_*(chr($cp)) should be
    used.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- Date:
  . The date_sunrise() and date_sunset() functions have been deprecated in
    favor of date_sun_info().
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . The strftime() and gmstrftime() functions have been deprecated in favor of
    date()/DateTime::format() (for locale-independent formatting) or
    IntlDateFormatter::format() (for locale-dependent formatting).

- Filter:
  . The FILTER_SANITIZE_STRING and FILTER_SANITIZE_STRIPPED filters have been
    deprecated.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . The filter.default ini setting is deprecated.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- GD:
  . The $num_points parameter of image(open|filled)polygon has been deprecated.

- Hash:
  . The mhash(), mhash_keygen_s2k(), mhash_count(), mhash_get_block_size() and
    mhash_get_hash_name() functions are deprecated. Use the hash_*() APIs
    instead.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- IMAP:
  . The NIL constant has been deprecated. Use 0 instead.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- Intl:
  . Calling IntlCalendar::roll() with bool argument is deprecated. Pass 1 and -1
    instead of true and false respectively.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- Mbstring:
  . Calling mb_check_encoding() without an argument is deprecated.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- MySQLi:
  . The mysqli_driver::$driver_version property has been deprecated. The driver
    version is meaningless as it hasn't been updated in more than a decade. Use
    PHP_VERSION_ID instead.
  . Calling mysqli::get_client_info in OO style or passing $mysqli argument to
    mysqli_get_client_info() function has been deprecated. Use
    mysqli_get_client_info() without any arguments to obtain the client
    library version information.
  . The mysqli::init() method has been deprecated. Replace calls to
    parent::init() with parent::__construct().
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- OCI8:
  . The INI directive oci8.old_oci_close_semantics has been deprecated.

- ODBC:
  . odbc_result_all() has been deprecated.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- PDO:
  . The PDO::FETCH_SERIALIZE mode has been deprecated.
    RFC: https://wiki.php.net/rfc/phase_out_serializable

- PgSQL:
  . Not passing the connection argument to PgSQL functions and using the
    default connection is deprecated.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- SOAP:
  . The ssl_method option for the SoapClient constructor has been deprecated in
    favor of ssl stream context options. The direct equivalent would be
    crypto_method, but min_proto_version/max_proto_version are recommended
    instead.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

- Standard:
  . Calling key(), current(), next(), prev(), reset(), or end() on objects
    is deprecated. Instead cast the object to array first, or make use of
    ArrayIterator.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . The strptime() function has been deprecated. Use date_parse_from_format()
    instead (for locale-independent parsing) or IntlDateFormatter::parse() (for
    locale-dependent parsing).
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . The auto_detect_line_endings ini setting has been deprecated. If necessary,
    handle "\r" line breaks manually instead.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1
  . The FILE_BINARY and FILE_TEXT constants are deprecated. They already had
    no effect previously.
    RFC: https://wiki.php.net/rfc/deprecations_php_8_1

========================================
5. Changed Functions
========================================

- Core:
  . Properties order used in foreach, var_dump(), serialize(), object comparison
    etc. was changed. Now properties are naturally ordered according to their
    declaration and inheritance. Properties declared in a base class are going
    to be before the child properties. This order is consistent with internal
    layout of properties in zend_object structure and repeats the order in
    default_properties_table[] and properties_info_table[]. The old order was
    not documented and was caused by class inheritance implementation details.

- Filter:
  . The FILTER_FLAG_ALLOW_OCTAL flag of the FILTER_VALIDATE_INT filter now accept
    octal string with the leading octal prefix ("0o"/"0O")
    RFC: https://wiki.php.net/rfc/explicit_octal_notation

- GMP:
  . All GMP function now accept octal string with the leading octal prefix ("0o"/"0O")
    RFC: https://wiki.php.net/rfc/explicit_octal_notation

- PDO ODBC:
  . PDO::getAttributes() with PDO::ATTR_SERVER_INFO and PDO::ATTR_SERVER_VERSION
    now return values instead of throwing PDOException.

- Reflection:
  . ReflectionProperty::setAccessible() and ReflectionMethod::setAccessible()
    no longer have an effect. Properties and methods are always considered
    accessible through reflection.
    RFC: https://wiki.php.net/rfc/make-reflection-setaccessible-no-op

- Standard:
  . syslog() is now binary safe.

========================================
6. New Functions
========================================

- Core:
  . Added array_is_list(array $array), which will return true if the array keys are 0 .. count($array)-1 in that order.
    RFC: https://wiki.php.net/rfc/is_list

- pcntl:
  . Added pcntl_rfork for FreeBSD variants

- Reflection:
  . Added ReflectionFunctionAbstract::getClosureUsedVariables

- Standard:
  . Added fsync() and fdatasync(), which instruct the operating system to
    flush its buffers to physical storage.
    RFC: https://wiki.php.net/rfc/fsync_function

- Sodium:
  . Added the XChaCha20 stream cipher interface functions:

    - sodium_crypto_stream_xchacha20()
    - sodium_crypto_stream_xchacha20_keygen()
    - sodium_crypto_stream_xchacha20_xor()

  . Added the Ristretto255 functions, which are available in libsodium 1.0.18.

    Ristretto is a technique for constructing prime order elliptic curve groups with non-malleable encodings.
    Ristretto255 implements Ristretto atop Curve25519.

    - sodium_crypto_core_ristretto255_add()
    - sodium_crypto_core_ristretto255_from_hash()
    - sodium_crypto_core_ristretto255_is_valid_point()
    - sodium_crypto_core_ristretto255_random()
    - sodium_crypto_core_ristretto255_scalar_add()
    - sodium_crypto_core_ristretto255_scalar_complement()
    - sodium_crypto_core_ristretto255_scalar_invert()
    - sodium_crypto_core_ristretto255_scalar_mul()
    - sodium_crypto_core_ristretto255_scalar_negate()
    - sodium_crypto_core_ristretto255_scalar_random()
    - sodium_crypto_core_ristretto255_scalar_reduce()
    - sodium_crypto_core_ristretto255_scalar_sub()
    - sodium_crypto_core_ristretto255_sub()
    - sodium_crypto_scalarmult_ristretto255()
    - sodium_crypto_scalarmult_ristretto255_base()

========================================
7. New Classes and Interfaces
========================================

- Intl:
  . Added IntlDatePatternGenerator to dynamically generate patterns to use with IntlDateFormatter.
    RFC: https://wiki.php.net/rfc/intldatetimepatterngenerator

========================================
8. Removed Extensions and SAPIs
========================================

========================================
9. Other Changes to Extensions
========================================

- GD:
  imagewebp() can do lossless WebP encoding by passing IMG_WEBP_LOSSLESS as
  quality. This constant is only defined, if a libgd is used which supports
  lossless WebP encoding.

- Mbstring:
  . Unicode data tables have been updated to Unicode 14.

- MySQLi:
  . The mysqli_stmt::next_result() and mysqli::fetch_all() methods are now
    available when linking against libmysqlclient.

- OpenSSL:
  . The OpenSSL extension now requires at least OpenSSL version 1.0.2.
  . OpenSSL 3.0 is now supported. Be aware that many ciphers are no longer
    enabled by default (part of the legacy provider), and that parameter
    validation (e.g. minimum key sizes) is stricter now.

- Phar:
  . Use SHA256 by default for signature.
  . Add support for OpenSSL_SHA256 and OpenSSL_SHA512 signature.

- SNMP:
  . add SHA256 and SHA512 for security protocol.

- Standard:
  . --with-password-argon2 now uses pkg-config to detect libargon2. As such,
    an alternative libargon2 location should now be specified using
    PKG_CONFIG_PATH.
  . --with-external-libcrypt now allows to use system libcrypt/libxcrypt
    instead in PHP own implementation.

========================================
10. New Global Constants
========================================

- MySQLi:
  . MYSQLI_REFRESH_REPLICA has been added as a replacement for
    MYSQLI_REFRESH_SLAVE, in line with an upstream change in MySQL. The old
    constant is still available for backwards-compatibility reasons, but may
    be deprecated/removed in the future.

- Sockets:
  . The following socket options are now defined if they are supported:
     * SO_ACCEPTFILTER
     * SO_DONTTRUNC
     * SO_WANTMORE
     * SO_MARK
     * TCP_DEFER_ACCEPT

========================================
11. Changes to INI File Handling
========================================

- The log_errors_max_len ini setting has been removed. It no longer had an
  effect since PHP 8.0.

- A leading dollar in a quoted string can now be escaped: "\${" will now be
  interpreted as a string with contents `${`.

- Backslashes in double quoted strings are now more consistently treated as
  escape characters. Previously, "foo\\" followed by something other than a
  newline was not considered as a teminated string. It is now interpreted as a
  string with contents `foo\`. However, as an exception, the string "foo\"
  followed by a newline will continue to be treated as a valid string with
  contents `foo\` rather than an unterminated string. This exception exists to
  support naive uses of Windows file pahts as "C:\foo\".

========================================
12. Windows Support
========================================

========================================
13. Other Changes
========================================

========================================
14. Performance Improvements
========================================