PHP网页重定向。 本地主机

Question

I was making a IMS that the admin must login then redirect the web page to the main page, but it does not it continue to loop, infinite looping, the error says localhost has too many redirects..

This is my index code:

<?php
session_start();
if(!empty($_SESSION['KEY']))
{
header("location: datagrid_customer_info.php");

}
if(isset($_SESSION['blocked']))
{
header("location: blocked.php");
}
?>

This is my login page code: "I did not include the html code"

<?php
session_start();
if(!isset($_SESSION['KEY']))
{
header("location:datagrid_customer_info.php");
}
?>
<?php 
if(isset($_POST['btnLogin'])){
    require("_config.php");

    $txtEmail = $_POST['txtEmail'];
    $txtPassword = $_POST['txtPassword'];

    $STH = $DBH->prepare("SELECT * FROM adm_login_info WHERE   
    cEmail=:txtEmail AND cPassword=:txtPassword");   
    $STH->bindparam(":txtEmail",$txtEmail);
    $STH->bindparam(":txtPassword",$txtPassword);
    $STH->execute();
    $ROW = $STH->fetch(PDO::FETCH_ASSOC);
    $CTR = $STH->rowCount();
        if($CTR==1){
            $_SESSION['idLogin'] = $ROW['idLogin'];
            $_SESSION['KEY'] = 1;

            $STH = $DBH->prepare("UPDATE adm_login_info SET cLock=0, 
            cLastLogin = DATE_FORMAT(NOW(),'%Y-%m-%d %H:%i:%s') WHERE 
            idLogin=:idLogin");
            $STH->bindparam(":idLogin",$ROW['idLogin']);
            $STH->execute();

            echo "<script type='text/javascript'>";
            echo "alert('Welcome to Inventory Management System');";
            echo "window.location.href='datagrid_customer_info.php';";
            echo "</script>";
        }else{
            echo "<script type='text/javascript'>";
            echo "alert('Login Error!');";
            echo "window.location.href='login.php';";
            echo "alert('INVALID USERNAME OR PASSWORD!!!!!');";
            echo "</script>";
        }   
}
?>

This is the main page code: "I also did not include the html, only the php code"

<?php
session_start();
if(empty($_SESSION['KEY']))
{
header('location: login.php');
}
?>
<?php
require_once("_config.php");
if(isset($_REQUEST['del_id']))
{

 $STH=$DBH->prepare("DELETE FROM inv_customer_info WHERE idCustomer=:id");
 $STH->bindparam(":id",$_REQUEST['del_id']);
 $STH->execute();
 header("Location: datagrid_customer_info.php");
}
?>