Php Update MySQL Database

        $server = "localhost";
        $db = "mysql";
        $user_name  = "xchatdemo";
        $pass = "adminatxchat";
        $db_tab = "xchat";
        $link = mysqli_connect("$server", "$user_name", "$pass", "$db");
        if (!$link) {
            echo "error***Unable to connect to server!";
            exit;
        }
            $sql = "UPDATE xchat SET full='$_full', dob='$_dob', loc='$_loc' WHERE username='$_username' AND pass='$_password';"; //VARIABLES COMES FROM $_GET METHOD THROUGH MY APPLICATION
            //if ($link->query($sqltwo) === TRUE) //TRIED THIS TOO!
            if (mysqli_query($link, $sql)) //THIS IS ALWAYS TRUE, EVEN WHEN THE WHERE CLAUSE DOES'NT MATCH.
            {
                echo "success***Your account is updated!"; //THIS STATEMENT EXECUTES EVEN THE DATABASE IS NOT UPDATED
            }
            else
            {
                echo "error***Error updating your account: Incorrect Password!"; //I WANT TO EXECUTE THIS QUERY IF USER AND PW IS INCORRECT
            }
            $link->close();