Environment: PHP 5.3.6, Zend Framework 1.11.6
This problem occurred when I needed to build a mixed environment where the existing code was "pure PHP" and new code was based on Zend Framework.
The "pure PHP" code's __autoload function stopped working as soon as Zend application ran.
Apparently, when there is a call to spl_autoload_register, __autoload is ignored.
The solution was to rename __autoload to OldAutoLoad and call:
Note: after that, the OldAutoload was called first and then Zend's autoload. In order to let OldAutoload ignore Zend classes, I replaced require_once calls with @include_once.