If you are getting PHP memory errors or perhaps Internal Server 500 error messages, your WordPress install may have exhausted or exceeded the current memory allocation of your server. Luckily, WordPress gives us an easy way to try to increase the PHP memory allocated on your sever.
Add the following line to your wp-config.php file before the include of wp-settings.php:
This attempt to increase memory will only work if your server allows it and is easier than modifying your php.ini or .htaccess file. If it doesn’t work, you will need to contact your host directly.
Also, depending on the complexity of your WP theme and the number of plugins installed, you may need more than 128MB.