Php is setup is to limit memory usage per process. If you require more, this limit can be increased.
Edit
/usr/local/lib/php.ini
If you are unsure about the php.ini path, You can find your server php.ini using command :
php -i | grep php.ini
Configuration File (php.ini) Path => /usr/local/lib
Loaded Configuration File => /usr/local/lib/php.ini
and set:
memory_limit = 8M ; Maximum amount of memory a script may consume (8MB)
to a higher value, like 20M. Save, exit, then restart apache.