//if user hit sumbit
    //check if field are not empty
    if(isset($_POST["username"]) && isset($_POST["password"]))
        $manager = preg_replace(#[^A-Za-z0-9#i',",$_SESSION["username"]]);  
        $password = preg_replace(#[^A-Za-z0-9#i',",$_SESSION["password"]]);

        include "../storescript/connect_to_mysql.php";

        $sql = mysql_query("SELECT id FROM admin WHERE username='$manger' AND password='$password' LIMIT 1");

        //make sure person exists in database---
        $existCount = mysql_num_rows($sql); //count the row num
        if($existCount == 1) //make sure name is only one in database
            while($row = mysql_fetch_array($sql))
                $id = $row["id"];

Parse error: syntax error, unexpected ';' in C:\xampp\htdocs\E-COMMERCE\admin\admin_login.php on line 20
line 20 is include "../storescript/co.....";

$manager = preg_replace(#[^A-Za-z0-9#i',",$_SESSION["username"]]); // Single Quotes are closed but not opened. CHECK!
$password = preg_replace(#[^A-Za-z0-9#i',",$_SESSION["password"]]);// Same thing as per previous line. Single Quotes Closed but not Opened