Added basic parsing to the check of realpath_cache_size

The existing code has a bug where it does not recognise values that use PHP's 'k' or 'm' syntax, e.g. "5k" = "5000".

To fix this, the pull request adds basic parsing to the realpath_cache_size value check to handle this.

In these cases it will always be at least 1k (i.e. 1000) which is high enough to pass the check, so the function returns true.