The new custom object serialization mechanism RFC introduced new __serialize() and __unserialize() magic methods in PHP 7.4, with the intent of replacing the broken Serializable interface. This RFC finalizes that work by laying out a plan for the eventual removal of Serializable.
Please see the referenced RFC for a detailed discussion of why the Serializable interface is broken and needs to be replaced. Since PHP 7.4 a robust alternative mechanism exists, but some of the motivating issues will only be resolved once support for Serializable is dropped entirely.
A class is “only Serializable” if it is non-abstract, implements Serializable, and does not implement __serialize() and __unserialize(). Then:
Serializable will be accepted without a deprecation warning, because libraries supporting PHP < 7.4 will generally need to implement both the old and new mechanisms.Serializable interface will be removed and unserialize() will reject payloads using the C serialization format. Code needing to support both PHP < 7.4 and PHP >= 9.0 may polyfill the Serializable interface, though it will have no effect on serialization.
If a class implements both Serializable and __serialize()/__unserialize(), the latter take precedence (on versions that support them), and the Serializable interface is only used to decode existing serialization payload using the obsolete C format. To migrate to the new mechanism, it's possible to either replace Serializable entirely (if support for PHP 7.3 and below is not needed) or to implement both (if it is needed).
An earlier version of this RFC proposed an additional step: PHP 9.0 would deprecate all uses of Serializable (including those that are not “only Serializable”) and only remove the interface in PHP 10.0. However, this approach was deemed too complicated.
PDO has a PDO::FETCH_SERIALIZE flag that can be used in conjunction with PDO::FETCH_CLASS. This fetch mode is based on the Serializable interface, and as such it cannot be supported once it is removed. Apparently, the PDO::FETCH_SERIALIZE mode is not actually usable due to an implementation bug (https://bugs.php.net/bug.php?id=68802) anyway.
In addition to the Serializable changes, this RFC proposes to deprecate PDO::FETCH_SERIALIZE in PHP 8.1 and remove it in PHP 9.0.
Voting started 2021-04-14 and ended 2021-04-28.