For those of you itching to test out php 5.1.0 here is how my testing went. 1) php 5.1.0 has completedly rewritten all the data functions. As such, you MUST add the following in your php.ini to avoid errors/warnings when using any of the date methods. Basically, 5.1.0 requires you to set the timezone manually as it doesn't the trust the one from the system. Code: date.timezone = "America/Los_Angeles" You can set it within your code as well. Check php.net for the all the timezone codes. 2) 5.1.0 also made some changes to its object code so you should test all your code to make sure nothing funky shows up. In my testing one snip of code involving custom object threw an interesting cannot convert to int error in a simple compare object to string "" or to null statement. I modified that line to use the empty() method instead. 3) APC 3.0.8 works well with 5.1.0. However, you must compile apc with Code: ./configure --enable-apc Do not use any of the --enable-apc-XXX options. The nmap option crashes php and the shm option leaks semaphores. Stick with "--enable-apc" only. 4) Latest 2.5.10a Zend Optimizer does not work with php 5.1.0. If you are using encrypted php code that requires zend manager/optimzer do not upgrade. So far..very stable. Have fun.