function?update($table,$array,$where=null){
	$link=mysqli_connect(DB_HOST,DB_USER,DB_PWD,DB_DBNAME)?or?die("數據庫連接失敗Error:".mysql_errno().":".mysql_error());
	mysqli_set_charset($link,DB_CHARSET);
	foreach($array?as?$key=>$val){
		if?($str==null){
			$sep="";
		}else{
			$sep=",";
		}
		$str.=$sep.$key."='".$val."'";
	}
		$sql="update?{$table}?set?{$str}?".($where==null?null:"?where?".$where);
		$result=mysqli_query($link,$sql);
		//var_dump($result);
		//var_dump(mysql_affected_rows());exit;
		if($result){
			return?mysqli_affected_rows();
		}else{
			return?false;
		}
}

$str是什么？