Wrap crypt_random_string in if !function_exists

Composer's "file" autoloader does a require instead of a require_once. This means that if you have two composer autoloaders in your project (common in Laravel 4 projects with workbenches) the function file is loaded twice and you get a fatal error of "cannot redeclare function".

Personally I prefer to wrap this in an if (...): endif; but using brackets is what complies with your codesniffer standards, so I updated to use that instead.

This is really only an issue with modern projects using composer, so if you'd rather I requested towards the PHP5 branch, please let me know.