PHP's call_user_func_array() is slow

I just solved an issue that had me waking up in the middle of the night for weeks (just pushing it a bit) and I have to thank PHP's community and in particular a guy named Brad Proctor for writing a very short but very important comment in the PHP manual, and I quote:
This function is relatively slow (as of PHP 5.3.3) and if you are calling a method with a known number of parameters it is much faster to call it this way: $class->{$method}($param1, $param2);
He mentions

Memcache(d) to store PHP sessions

Edit: as of July 2014, there is a recent article discussing the use of php5-memcache instead of php5-memcache to store sessions on this blog. There are many posts around on the web about using memcached to store sessions data on high-availability servers, but few actually cover the whole topic and there are some elements that I thought might be of interest in the form of a quick recap. Memcached allows you to store frequently-used data in memory (RAM).