php memory_limit issue

[Silh]

one of my script required large memory. and probably because of this I am getting following error\[quote\] Fatal error: Out of memory (allocated 42729472) (tried to allocate 32 bytes)\[/quote\]I tried to change memory_limit by using ini_set('memory_limit', '256M'); on the top of my page but still I am getting same issues. I also tried ini_set('memory_limit', '512M'); but no luck.Can anyone help me to solve this?thanks