The deprecation message was originally introduced in 3e6b447 (#6494).
I first encountered this notice when testing the MongoDB extension
with PHP 8.1, which produced many duplicate messages that provided
no detail about the particular class that needed to be fixed.
Closes GH-7346.
If Serializable is implemented, require that __serialize() and
__unserialize() are implemented as well, else issue a deprecation
warning.
Also deprecate use of PDO::FETCH_SERIALIZE.
RFC: https://wiki.php.net/rfc/phase_out_serializable
Closes GH-6494.
