unexpected T_STRING, expecting T_VARIABLE 深夜未睡的大神们，靠你们了！该怎么处理

unexpected T_STRING, expecting T_VARIABLE 深夜未睡的大神们，靠你们了！
Parse error: syntax error, unexpected T_STRING, expecting T_VARIABLE in C:\AppServ\www\vote\db.php on line 13
部分代码：
class db{
private static $dbhost;
  private static $dbuser;
private static $password;
private static $dbname;
private static $instance=NULL;

private function_construct($host,$user,$pass,$db){ //line 13
  self::$dbhost = $host;
self::$dbuser = $usr;
self::$password = $pass;
self::$dbname = $db;
self::creatcon();
}
为什么会错？为什么？求教！

------解决方案--------------------
private function __construct($host,$user,$pass,$db){

空格 双下划线。