A common optimization that can be done for PHP to improve performance is to install Alternative PHP Cache (APC).
PHP is a dynamically compiled language: every file has to be parsed by the interpreter and compiled down to opcode which is then translated into cpu-executable machine bytecode. The time needed for these steps, for each and every executed php file makes it slow from a performance perspective. PHP accelerators (aka opcode caching) keep a copy of the compiled script (bytecode) in memory– so you save 2 steps out of 3 (Read/Compile/Execute.)
APC is arguably the most popular opcode cache (others include XCache & eAccelerator.) If you need to install APC, there are myriad tutorials on the internet and it is quite easy. You can check your phpinfo() to see it is installed properly.
The next and more important step is to configure your APC. Badly set up it could make your website even slower. Here is the configuration that you can define in your apc.ini or php.ini depending of your installation.
Enable or disable APC