Php is setup is to limit memory usage per process. If you require more, this limit can be increased.
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
memory_limit = 8M ; Maximum amount of memory a script may consume (8MB)
to a higher value, like 20M. Save, exit, then restart apache.