The PHP memory_limit is a legacy option that applied only to PHP 5.3 and earlier. This option is no longer used.
If you are still using PHP 5.3 for some reason, don't. It was discontinued years ago, is insecure, and its use is not supported in any form or fashion, including raising the hardcoded memory limit. If you encounter memory issues... or any other issues... with a PHP 5.3 site, the solution is to update the site to a supported version of PHP.
If you are using any supported version of PHP, you do not need to raise the per-process memory limit because there isn't one.