code snippet(debug)

The problem is, you're not specifying and using the connection handler anywhere in test.php. Return the connection handler from the startcon() method and use it in mysql_query() method. Below is the working code.

Sidenote: Please don't use mysql_ database extensions, they are deprecated as of PHP 5.5 and are removed altogether in PHP 7.0. Use mysqli or PDO extension instead.


connection.php

<?php
    class DBcon
    {
        // property declaration
        public $ip = "localhost";
        public $user = "root";
        public $pass = "";
        public $db = "stackoverflow";

        // method declaration
        public function startcon()
        {
            $con=mysql_connect($this->ip,$this->user,$this->pass);
            mysql_select_db($this->db, $con);
            return $con;
        }
    }
?>



test.php

<?php
    include_once("connection.php");

    $c = new DBcon();
    $connection = $c->startcon();
    global $array ;

    $s = "SELECT  B.question, B.ansSel FROM quizcheck B
        WHERE EXISTS
        (SELECT * FROM quizes A
        WHERE A.question = B.question AND A.Answer = B.ansSel)";
    $result = mysql_query($s, $connection);

    $r = mysql_fetch_array($result) or die(mysql_error());

    if(mysql_num_rows($r) >= 1){
        echo "correct";
    }else{
        echo "list of wrongs";
    }
?>