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I've added several tests to confirm existing behaviour of the is_serialize() function. I've based the input for these tests on the existing tests for is_serialized_string.

To test the Enum serialization, I've created and serialized an Enum on 3v4l.org ((link)[https://3v4l.org/J6h8o]). The given input and result there are shown below for PHP 8.1, which confirms the case for the E character that's added in the is_serialize() function.

<?php

enum Foo
{
    case bar;
}

echo serialize( Foo::bar );
// E:7:"Foo:bar";

committed in https://core.trac.wordpress.org/changeset/53886