src/AbstractTransport.php
public static function allowedMethod($method)
    {
        if (is_string($method) && ($method = strtoupper(trim($method))) != '') {
            return defined(static::class . '::METHOD_'. $method) ? $method : false;
        }

        return false;
    }
public static function allowedMethod($method)
    {
        if (is_string($method) && ($method = strtoupper(trim($method))) != '') {
            return defined(static::class . '::METHOD_'. $method) ? $method : false;
        }

        return false;
    }
Method Allowed

@param string $method
@return bool|string the method