added switchable warning for comparison numbers and strings

[i582]

In PHP 8, the logic for comparing numbers and strings has changed, so we need a way to find all such places, while not including warning at all, as this will lead to a huge number of warnings.

In order to enable the warning, the set_migration_php8_warning function has been added, with which you can enable or disable the warning. This function accepts a bitset, in which only the first right bit is used so far, but the rest will be used in the future.

<?php
$b = 0;

set_migration_php8_warning(0b001);

if ($b == "") {} // warning

set_migration_php8_warning(0b000);

if ($b == "") {} // no warning

At the end of each request, the warning is automatically disabled.

Changes in

The operators <=>, ==, !=, >, >=, <, and <=.
The functions in_array(), array_search() and array_keys() with $strict set to false (which is the default).
The sorting functions sort(), rsort(), asort(), arsort() and array_multisort() with $sort_flags set to SORT_REGULAR (which is the default).

Links

RFC: https://wiki.php.net/rfc/string_to_number_comparison

In documentation: String to Number Comparison

The following table shows how the result of some simple comparisons changes (or doesn't change) under this RFC:

Comparison	Before	After
0 == "0"	true	true
0 == "0.0"	true	true
0 == "foo"	true	false
0 == ""	true	false
42 == " 42"	true	true
42 == "42foo"	true	false

See full table in tests:

kphp/tests/cpp/runtime/number-string-comparison.cpp

Line 21 in d5a76ad

TEST(number_string_comparison_test, test_comp_int) {

Note

We are already comparing infinities as in PHP 8.
See Special values part of RFC.

Related changes (not implemented in this PR, see #298)

In https://wiki.php.net/rfc/saner-numeric-strings (implemented in PHP 8):

42 == "42"; // true
42 == "   42"; // true
42 == "42   "; // true

Before

42 == "42"; // true
42 == "   42"; // true
42 == "42   "; // false

See VKCOM/kphp-polyfills#25
#290

[i582]

Closed due #298
Thшы two PRs are merged into one since the second depends on the first.