How do you edit/modify the mem allocation for PHP?

I have been trying to install a few things and keep getting Smarty errors 
stating: 

Fatal error: Allowed memory size of 8388608 bytes exhausted (tried to allocate 
35 bytes) in /var/www/html/tikiwiki_1.7.1.1/Smarty/Smarty_Compiler.class.php 
on line 255

What file needs to be modified to allow a larger memory allocation?

Thanks!

Chris