Cache.DefinitionImpl / Cache.SerializerPath ignored

[sogerc1]

I have HTMLPurifier 4.15.0 from PEAR on an ubuntu server 22.04.

I've set
$config->set('Cache.DefinitionImpl', null);
but I'm still seeing this warning:

/usr/share/php/HTMLPurifier/DefinitionCache/Serializer.php(297): Directory /usr/share/php/HTMLPurifier/DefinitionCache/Serializer not writable, please chmod to 777

If I add:
$config->set('Cache.SerializerPath', '/some/dir'); // which is writable by the web server
the exact same warning still appears (still mentioning DefinitionCache/Serializer not my directory).

I can't set the permissions of the default directory because I have no access and I don't need caching anyway because I'm doing inbound filtering.

[sogerc1]

Additional info: even though I have those two options cache files still appear in the DefinitionCache/Serializer directory if I give write permission in my development virtual machine.