Only echo works in this code in this if statement (PHP) - php

Question

$con = mysqli_connect('localhost', 'login', '*mypass*', 'login');
$check = "SELECT * FROM users";
$rs = mysqli_query($con, $check);
if(isset($_POST['submit'])) {
$username = strtolower($_POST['username']);
$password = hash('sha512', $_POST['password']);
if($username && $password){
while($data = mysqli_fetch_array($rs)){
if($username == $data[1]){
if($password == $data[2]){
session_start();
$_SESSION['loggedin'] = true;
$_SESSION['username'] = $username;
$exp = time() + (60*60*24*365);
setcookie('user', $username, $exp);
echo "test";
header("location: dash.php");
exit();
} else {
error("Incorrect Password");
}
}
}
} else {
error("Please insert your username and password");
}
}
In this code, the content of the if statement ( if($password == $data[2]) ) doesn't run correctly: only the echo function works, all the other doesn't!
Is there a solution to this issue?
I use PHP 7.0
POST:
echo "test"; was added only for testings purposes, the code doesn't work with or without it.

Answer 1

Looking at this part:
if($username && $password){
while($data = mysqli_fetch_array($rs)){
if($username == $data[1]){
if($password == $data[2]){
You should be careful when reading $data. Put simply, the $data array is indexed on a per-row basis starting from 0, so $data[1] and $data[2] refer to the entire second and third resultset rows, but you're not actually reading any result fields.
The right and cleaner way would be:
if($username && $password){
$data = mysqli_fetch_array($rs);
foreach ($data as $row)
{
$username = $row['field_name_with_username];
$password = $row['field_name_with_password];
... then whatever you need to do afterwards with those variables
}
If you rather stay away from the foreach looping, you can use while or for, but you need to define an interation counter (i.e. $i = 0; before entering the loop and $i++; just before the end of each iteration) and on each pass access $data[$i]['field_name_with_username'] and $data[$i]['field_name_with_password']

Answer 2

This is not an answer but something to try to check sessions are working.
Place the following code in its own file:
<?php
session_start();
if(isset($_SESSION['test_counter'])) {
$_SESSION['test_counter']++;
} else {
$_SESSION['test_counter'] = 1;
}
echo $_SESSION['test_counter'];
It should first display 1, and then increment the integer upon refresh.