There are primarily two clients used with PHP. One is the older, more widespread pecl/memcache and the other is the newer, less used, more feature rich pecl/memcached.
Both support the basics such as multiple servers, setting vaules, getting values, increment, decrement and getting stats.
Here are some more advanced features and information.
https://code.google.com/p/memcached/wiki/PHPClientComparison