Will be prefer to change "kphp-polyfills" licence to LGPL (GPL for libraries)
Because it redistributable package like library and should not affect application licence.

GPL3- after using package kphp-polyfills in application, the application must apply GPL3 conditions and support GPL licence.

Potential fix:

if (!function_exists('msgpack_serialize')) {
  function msgpack_serialize(...) {
  }
}

• Same double declaration guards against other msgpack functions.

kphp actual msgpack_serialize returns ?string, but

main();

function main() {
  instance_deserialize(instance_serialize(new Foo()), Foo::class);
}

/** @kphp-serializable */
class Foo {

  /** @kphp-serialized-field 1 */
  public ?Bar $bar = null;
}


/** @kphp-serializable */
class Bar {

  /** @kphp-serialized-field 1 */
  public int $v = 1;
}

This code fails in runtime with an error saying that class Bar can not be found. If the type name is moved from type hint to PHPDoc everything works fine.