Why is $_SERVER[\'HTTP_X_REQUESTED_WITH\'] Missing from the Official PHP Documentation, and Is It Reliable?

Does PHP Lack $_SERVER['HTTP_X_REQUESTED_WITH'?

Introduction

Developers commonly employ $_SERVER['HTTP_X_REQUESTED_WITH'] to distinguish between AJAX and non-AJAX requests. However, the official PHP documentation fails to list this variable, leading to doubts about its existence.

Question

Why is $_SERVER['HTTP_X_REQUESTED_WITH'] absent from the official PHP documentation, and does it actually exist?

Answer

The Origin of $_SERVER Variables

The variables within $_SERVER originate from the web server, not PHP itself. Consequently, they are not documented in the PHP documentation.

$_SERVER['HTTP_X_REQUESTED_WITH'] Functionality

This variable generally reflects the X-Requested-With header sent by Ajax functions of major frameworks. However, certain frameworks, like Dojo, recently added this header.

Reliability Limitations

$_SERVER['HTTP_X_REQUESTED_WITH'] is not a completely reliable indicator of AJAX requests as not all frameworks consistently send the X-Requested-With header.

Alternative Solution

For absolute certainty, send a pre-defined flag (e.g., a GET variable) with the request and check its presence on the receiving page to determine if it's an AJAX request.

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