Paul Reinheimer has a reminder shown in his latest post about optimizing your scripts - don't forget the basics like unset.

A colleague called me over today for some help with a memory usage issue in his PHP script. The script was performing the basic (but critical) task of importing data, pulling it in from MySQL in large chunks, then exporting it elsewhere. He was receiving the wonderful "Fatal error: Allowed memory size of XXXX bytes exhausted (tried to allocate YY bytes)" error message. [...] I fixed the memory usage exceeded problem with an unset(), at the end of a loop.

The loop was structured such that the large amounts of data were just being held in memory over and over again, resulting in a huge stackup that can easily cause the "Allowed memory size exhausted..." message.

There are better architectures available to avoid the issue entirely (like not storing everything in the array, just to iterate over it later) but they're an issue for a different post. For a very simple base case demonstration of the issue take a look at the simple example.