Insert to table always failure - php

Question

Its a simple registration with username and password, but when I try to insert it to the database it always fails and I don't know why, can someone help me with this?
include "db.php";
if (isset($_POST['submit']))
{
$username= $_POST['leguser'] ;
$password= $_POST['legpass'] ;
$pwhash = password_hash($password, PASSWORD_DEFAULT) ;
$username = mysql_real_escape_string($username);
$password = mysql_real_escape_string($password);
echo var_dump ($username);
echo var_dump ($password);
echo var_dump ($pwhash);
$sql = "SELECT * FROM tbl_users WHERE fld_username = '$username'";
$result = $conn->query($sql);
if ($result->num_rows === 1) {
echo "<script>alert('Username already used!');</script>"; }
else
{
$q = "INSERT INTO `tbl_users` (`fld_username`) VALUES ('$username')";
$result = mysql_query($q);
if ($result) {
echo 'success';
} else {
echo 'failure';
}
}
This is the code of the database connection (db.php)
<?php
$DB_HOST = 'localhost';
$DB_USER = 'root';
$DB_PASS = '';
$DB_NAME = 'rsi_db';
$conn = new mysqli($DB_HOST, $DB_USER, $DB_PASS, $DB_NAME);
if($conn->connect_errno > 0) {
die('Connection failed [' . $conn->connect_error . ']');
}
By the way I only want to input only one data on the database which is the username, for me not to get complicated with every data that I want to put in the future.