PHP——mysql_connect(): The mysql extension is deprecated

PDO:
$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass);

MYSQLI:
$link = mysqli_connect(   
                'localhost',  /* The host to connect to 连接MySQL地址 */   
                'user',      /* The user to connect as 连接MySQL用户名 */   
                'password',  /* The password to use 连接MySQL密码 */   
                'world');    /* The default database to query 连接数据库名称*/   
      
    if (!$link) {   
       printf("Can't connect to MySQL Server. Errorcode: %s ", mysqli_connect_error());   
       exit;   
    }