ช่วยดูปัญหาการ INSERT ข้อมูล mssql_query แล้ว error "mssql_query(): supplied argument is not a valid MS SQL-Link resource in C:\AppServ\...."

function connectdb($db_name="database",$user="username",$pwd="password"){
		$this->database = $db_name;
		$this->username = $user;
		$this->password = $pwd;
		$this->connect_db = mssql_connect ( $this->host, $this->username, $this->password ) or $this->_error();
		//$this->connect_db = mysql_pconnect ( $this->host, $this->username, $this->password ) or $this->_error();
		$this->db = mssql_select_db ( $this->database, $this->connect_db) or $this->_error();
		//mssql_query("SET NAMES TIS620"); 
		//mssql_query("SET character_set_results=UTF8"); 
		return true; 
	}

$db->connectdb("erathai","sa","1234");
$sql = "INSERT INTO tm_customer (Cuscode,Note) VALUES ('$_POST[CusCode]','test') ";
$objQuery = mssql_query($sql,$db);
if($objQuery){
	echo "Save Done.";
}else{
	echo "Error Save [".$sql."]";
}