Call to undefined function mysql query with Login

Question

When I run this PHP code I am getting a fatal error.

Code

    <?php


    $user = 'root';

    $password = 'root';

    $db = 'inventory';

    $host = 'localhost';

    $port = 8888;


    $link = mysqli_init();

    $success = mysqli_real_connect(

       $link,

       $host,

       $user,

       $password,

       $db,

       $port

    );


    ?>

    <?php

    $username = $_POST['username'];

    $password = $_POST['password'];

    $sql = mysql_query("SELECT * FROM login WHERE username = '".$_POST['username']."' and password = '".md5($_POST['password'])."'");

    $row = mysql_num_rows($sql);

    if($rom > 0 )

    {

      session_start();

      $_SESSION['username'] = $_POST['username'];

      $_SESSION['password'] = $_POST['password'];

      echo "login done";

    }else {

      echo "fail login ";

    }

The Error

PHP Fatal error: Uncaught Error: Call to undefined function mysql_query() in /Applications/MAMP/htdocs/lprapp/config.php:23 Stack trace:#0 {main} thrown in /Applications/MAMP/htdocs/lprapp/config.php on line 23

Can someone help me with this issue?