PHP eAccelerator is a free open source PHP accelerator, optimizer and dynamic content cache module that specially build to optimize php performance. According to php eaccelerator official site, php accelerator able to speed up the php code by 1-10 times and reduce server load.
 
Installing PHP eAccelerator from source
Requirements
     * php4 or php5
    * autoconf
    * automake
    * libtool
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