Seems to be setting `$app_version = null` as a default and placing the `php_version()` in automatically would make for cleaner internal usage while letting applications override the version with custom strings when initializing their own http clients.

Something like:

	public static function qualifiedUserAgent($app_version = null) {
		return self::USER_AGENT . '(php ' . php_version() . ') ' . $app_version;
	}

Just a thought.