Description:
------------
Although we have a new Serialization mechanism (https://wiki.php.net/rfc/custom_object_serialization), this bug still persists on PHP 7,3 as:

When returning something different from an array when calling `serialize()` on a class that has the method `__sleep` implemented, the error message says that a `serialize(): __sleep` should return an array, instead of `Foo::__sleep()`.



Test script:
---------------
<?php
class Foo
{
    public function __sleep()
    {
        return 1;
    }
}

serialize(new Foo());

Expected result:
----------------
Notice: Foo::__sleep() should return an array only containing the names of instance-variables to serialize in %d on line %d


Actual result:
--------------
Notice: serialize(): __sleep should return an array only containing the names of instance-variables to serialize in %d on line %d