function get_bytes($mem_limit) {
    if(is_numeric($mem_limit)) {
        return $mem_limit;
    } else {
        $numeric_value = substr($mem_limit, 0, strlen($mem_limit) - 1);
        $unit_value = strtolower(substr($mem_limit, strlen($mem_limit) - 1));
        
        switch($unit_value) {
            case 'g':
                return $numeric_value * 1024 * 1024 * 1024;
                break;
            default:
            case 'm':
                return $numeric_value * 1024 * 1024;
                break;
            case 'k':
                return $numeric_value * 1024;
                break;
        }
    }
}

Example usage:

print get_bytes(ini_get("memory_limit"));

Fixed in 5.1 (Builds 2010082201+)