php admin session session - php

Question

I'm trying to make an admin account for my website using php. I'm using the following code and I get "500 internal Server Error" I have no idea what i'm doing wrong.
I have the following php script in my index.php file for admin.
<?php
session_start();
if(!isset($_SESSION["manager"])){
header("Location: admin_login.php");
exit();
}
$id = preg_replace('#[^0-9]#i', '', $_SESSION["id"]);
$manager = preg_replace('#[^0-9]#i', '', $_SESSION["manager"]);
$password = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["password"]);
include "../scripts/db_connect.php";
$sql_str = mysql_query("SELECT * FROM admins WHERE userName = '$userName' AND password = '$password' LIMIT 1");
$exist_Count = mysql_num_rows('$sql_str');
if($exist_Count == 0){
header('location: ../index.php');
exit();
}
?>
and the following code is for admin_login.php file where I ask the user to sign in
<?php
if(isset($_POST["userName"]) && isset($_POST["password"])){
$manager = $_POST["userName"];
$password = $_POST["password"];
include "../scripts/db_connect.php";
$results = mysql_query("SELECT id FROM admins WHERE userName = '$manager' AND password ='$password' LIMIT 1");
$existCount = mysql_num_rows($results);
if($existCount == 1){
while($row = mysql_fetch_array($results)){
$id = $row["id"];
}
$_SESSION["id"] = $id;
$_SESSION["manager"] = $manager;
$_SESSION["password"] = $password;
header("Location: index.php");
exit();
}
else{
echo 'Invalid Information';
exit();
}
}
?>

Answer 1

You forgot to add session_start() on your admin_login.php
<?php
session_start(); //<---------- Here
if(isset($_POST["userName"]) && isset($_POST["password"])){
$manager = $_POST["userName"];
$password = $_POST["password"];
include "../scripts/db_connect.php";
$results = ......
//.... rest of your code............