Today, I had to track down a bug in my application which only occured with PHP4. The problem was that I use PHP5 and I couldn’t reproduce the bug with PHP5. So I needed PHP4 on my machine. I installed php4-cgi, but for some reason I was not able to configure it correctly (I am not an Apache guru *g*). After more searching I found a hint of a special property in php.ini with which it is possible to run PHP5 in PHP4 mode. That property is:
zend.ze1_compatibility_mode
After enabling this property, I was able to reproduce the bug.

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