mysqli

<?php

    $server = 'localhost';
    $db_username = 'khalirium';
    $db_password = '';
    $db_name = 'blo';

    $conn = new mysqli($server, $db_username, $db_password, $db_name) or die("Connection failed: " . $conn->connect_error);

    $username = mysqli_real_escape_string($conn, $_POST['username']);
    $sql = "SELECT username FROM users WHERE username='$username'";

    if (mysqli_query($conn, $sql)) {
        //Return the number of rows in results set
        $rowcount=mysqli_num_rows($result);
        echo $rowcount;
    } else {
        echo 'No row';
    }

    mysqli_close($conn);
?>