A very excellent blog about PHP memory usage: http://blog.preinheimer.com/index.php?/archives/354-Memory-usage-in-PHP.html
Unlike C/C++ developers, most PHP developers never think about memory management in PHP. It is true that PHP can free the memory after a request is processed. However, if you use PHP to process critical tasks with large amount of data, you'd better read this blog carefully, and test its memory-usage-example.php by yourself. You will see how different it is.