Quick Question

[shutterbug]

I have created a form for quick additions to database. It has the following code at the front

<?php 
session_start();
if (!isset($_SESSION["manager"])) {
    header("location: admin_login.php"); 
    exit();
}
// Be sure to check that this manager SESSION value is in fact in the database
$managerID = preg_replace('#[^0-9]#i', '', $_SESSION["id"]); // filter everything but numbers and letters
$manager = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["manager"]); // filter everything but numbers and letters
$password = preg_replace('#[^A-Za-z0-9]#i', '', $_SESSION["password"]); // filter everything but numbers and letters
// Run mySQL query to be sure that this person is an admin and that their password session var equals the database information
// Connect to the MySQL database  
include "../SiteScripts/connect_to_mysql.php"; 
$sql = mysql_query("SELECT * FROM admin WHERE id='$managerID' AND username='$manager' AND password='$password' LIMIT 1"); // query the person
// ------- MAKE SURE PERSON EXISTS IN DATABASE ---------
$existCount = mysql_num_rows($sql); // count the row nums
if ($existCount == 0) { // evaluate the count
     echo "Your login session data is not on record in the database.";
     exit();
}
?>

It works fine for the first page but any other page it tells me "Your login session data is not on record in the database" If I take out the above code then it works fine. Any ideas on how to fix this issue? It use to work for all the pages but now it doesn't and I'm not sure what is wrong. Any ideas are much appreciated.