free allocated memory: HOW ?

Hello all, is there a way to free memory allocated by variables in PHP?? This is a very important issue concerning long running scripts... I have a script that generates > 5000 Newsletters and when the script finishes it uses 1.8 GB (!!) of RAM. Although I am using unset() to clean up variables (tried with $var = null too). I've read sth about php not giving the allocated memory back to the OS until a script finishes, is that right?? Is it possible to free the memory "by hand"? Any suggestion is appreciated! thx in advance