How to check in PHP whether page is directly loaded or being redirected from other page?

Suppose I am having two pages Page1.php and P开发者_运维知识库age2.php. Sometimes Page1.php is directly loaded and sometimes it is being redirected from Page2.php. My question is that can I check whether Page1.php is directly loaded or it's being redirected from the Page2.php.(if query string is not used)

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Set a session variable in Page2.php and control for it in Page1.php

Page2.php

<?php
session_start();
$_SESSION['from2'] = true;
header('Location: /Page1.php');
?>

Page1.php

<?php
session_start();
if(isset($_SESSION['from2']) && $_SESSION['from2']) {
  /* from Page2.php logic here */

  unset($_SESSION['from2']);
}else{
  /* not from Page2.php */
}
?>

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You can read the value of global variable $_SERVER['HTTP_REFERER'].

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$_SERVER['HTTP_REFERER']

Though check out the manual to see why it cannot be trusted: http://uk3.php.net/manual/ro/reserved.variables.server.php