While it is possible to exclude members from serialization using __sleep(), it has downsides:
- Using get_object_vars($this) to return the members is extremely slow
- Manually maintaining an array with the members is error prone and can be quite some work

A transient keyword to exclude members from serialization would make it easier ...

Using a transient keyword is better because if we have the following 
case:

class A { $a; $b; }
class B extends A { $c  function __sleep() { return array("c"); } }

now if you serialize b you will not serialize the $a and $b members from 
class A. So the sleep functions actually changes the encapsulated 
behaviour of A. Or we should call the superclass A::__sleep() method and 
join that with the new arrow.
However this is much cleaner code. And wether a member is transient or 
not should be declered nearby the member to keep else other developers 
may not now wether the member will be serialized or not.

Patch already exists, just need to discuss when is the correct time to add it.

http://whisky.macvicar.net/patches/php-transient.diff.txt

I think we should rather ease serialization customization. If we could simply adjust the object before serialization, use the default serialization, then adjust tje object after a default unserialization, we could solve this case by simply unsetting problematic properties before serialization.

> PHP has no advanced support for serialization.

PHP has already different ways to customize serialization, namely
__sleep()[1] and Serializable[2], the latter being superseeded by
__serialize()[3] as of PHP 7.4.0.  Adding support for the
transient keyword (or similar) *might* make sense, but that should
be discussed on the internals mailing list[4].  Feel free to start
that discussion; for the time being, I'm suspending this ticket.

[1] <https://www.php.net/manual/en/language.oop5.magic.php#object.sleep>
[2] <https://www.php.net/manual/en/class.serializable.php>
[3] <https://www.php.net/manual/en/migration74.new-features.php#migration74.new-features.standard.magic-serialize>
[4] <https://www.php.net/mailing-lists.php#internals>