Packer

Converts any PHP variable into a URL-safe string — and back. Supports HMAC signing, AES-256-CBC encryption, and gzip compression.

The Packer is particularly useful for:

• inserting a value into a URL,
• inserting a value into a hidden form field,
• inserting a value into a cookie.

In all cases, the value itself is securely hidden from the user and protected against unauthorized tampering.

Installation

composer require atk14/packer

Basic usage

// Pack
$packed = Packer::Pack(["user_id" => 42, "role" => "admin"]);
// e.g. "a1b2c3d4e5f6a1b2peyJ1c2VyX2..."

// Unpack
if (Packer::Unpack($packed, $data)) {
    echo $data["role"]; // "admin"
}

Every packed string is signed with HMAC-SHA256. Tampered or forged strings are rejected on unpack.

Configuration

Set a secret before using the library — either via a constant or SetSalt():

// Option A: define a constant (e.g. in bootstrap)
define("PACKER_CONSTANT_SECRET_SALT", "your-secret-here");

// Option B: set at runtime (can be changed per request)
Packer::SetSalt("your-secret-here");

Important: Never use the default salt ("Put_Some_Secret_Text_Here") in production. Anyone with access to the source code could forge valid packed strings.

Global defaults can also be set via constants:

Constant	Default	Description
PACKER_CONSTANT_SECRET_SALT	"Put_Some_Secret_Text_Here"	Secret for signing and encryption
PACKER_ENABLE_ENCRYPTION	false	Enable AES-256-CBC encryption
PACKER_USE_COMPRESS	false	Enable gzip compression
PACKER_USE_JSON_SERIALIZATION	true	Use JSON (true) or PHP serialize() (false)
PACKER_SIGNATURE_LENGTH	16	Number of Base64URL characters used as the HMAC-SHA256 signature (8–43)

Encryption

define("PACKER_CONSTANT_SECRET_SALT", "your-secret-here");

$packed = Packer::Pack($data, ["enable_encryption" => true]);

if (Packer::Unpack($packed, $data, ["enable_encryption" => true])) {
    // ok
}

Each call to Pack() with encryption generates a random IV, so the same input produces a different output every time.

Per-call password (extra_salt)

Useful when different users or sessions need isolated packed values:

$packed = Packer::Pack($data, [
    "enable_encryption" => true,
    "extra_salt"        => $user_token,
]);

Packer::Unpack($packed, $out, [
    "enable_encryption" => true,
    "extra_salt"        => $user_token, // must match
]);

Compression

$packed = Packer::Pack($large_array, ["use_compress" => true]);

Compression is detected automatically on unpack — no need to pass any option.

Decode() — inline unpacking

Decode() is a shorthand for Unpack() that returns the value directly instead of using an output parameter:

$data = Packer::Decode($packed);

To distinguish a failed unpack from a legitimately packed null, use the optional second parameter:

$data = Packer::Decode($packed, $ok);
if (!$ok) {
    // tampered or invalid string
}

Method reference

Packer::Pack($variable, $options = []): string

Packs a variable into a URL-safe string.

Option	Type	Default	Description
enable_encryption	bool	PACKER_ENABLE_ENCRYPTION	Encrypt with AES-256-CBC
use_compress	bool	PACKER_USE_COMPRESS	Compress with gzip
use_json_serialization	bool	PACKER_USE_JSON_SERIALIZATION	JSON vs PHP serialize
extra_salt	string	""	Additional secret for signing and encryption

Packer::Unpack($packed, &$out, $options = []): bool

Unpacks a string. Returns true on success, false if the string is invalid or tampered.

Accepts the same options as Pack(), except use_compress (auto-detected from the packed string).

Packer::Decode($packed, &$decoded = false): mixed

Shorthand for Unpack(). Returns the unpacked value, or null on failure. Sets $decoded to true/false.

Packer::SetSalt($salt): string

Sets the runtime secret salt. Returns the previous salt.

Security notes

• Signing: Every packed string is signed with HMAC-SHA256. Any modification to the string is detected on unpack.
• Encryption: AES-256-CBC with a random IV. The encryption key is derived from PACKER_CONSTANT_SECRET_SALT + runtime salt (set via SetSalt()) + extra_salt using SHA-256.
• Deserialization: When using PHP serialize() mode, objects are never instantiated on unpack (allowed_classes => false).

Requirements

• PHP >= 5.6
• Extensions: openssl, zlib (only needed if compression is used)

License

MIT