Core API

3.8.6. Warnings

There are a few warnings about using XCLASS extensions:

1. Avoid using XCLASS extensions in your (public) extensions!
   A  PHP class can only be extended by one extension class at a time. Thus, having two extension classes set up, only the latter one will be enabled. There is no way to work around this technologically in PHP. However "t3lib_div::makeInstance()" supports "cascaded" extension classes, meaning that you can do "ux_ux_someclass" which will extend "ux_someclass" but this requires an internal awareness of the extension class "ux_someclass" in the first place.
   The conclusion is that XCLASS extensions are best suited for project development where you need a quick hack of something in the core which should still stay backwards compatible with TYPO3 core upgrades.

2. Check if child classes are instantiated
   Quite often people have been confused about extending for instance the "tslib_menu" class when they want to add a feature for "TMENU". But actually the class to extend is "tslib_tmenu" which is an extension of "tslib_menu". So make sure you are extending the right class name (and always make sure your extension class is included also).

3. Strange opcode caching behaviours when you upgrade TYPO3 core
   When you upgrade the TYPO3 core and you have an extension which extends a core class, the upgraded core underneath might not be detected by opcode caches. In particular PHP-Accelerator is known for this behaviour producing "undefined function...." errors. The solution is: Always clear "/tmp/php_a_*" files and restart your webserver after upgrading source.