In PHP, the header() function is used to send an “Authentication Required” message to the client browser causing it to pop up a Username/Password input window. In fact header() allows you to send any raw HTTP header.

header(string $header, bool $replace = true, int $response_code = 0): void

The string parameter is passed to the header() function. For example

header("HTTP/1.1 404 Not Found");

It is used to figure out the HTTP status code to send.

You can also use header() function to redirect the browser to another URL.

Once the user has filled in a username and a password, the URL containing the PHP script will be called again with the predefined variables PHP_AUTH_USER, PHP_AUTH_PW, and AUTH_TYPE set to the user name, password and authentication type respectively. These predefined variables are found in the $_SERVER array. Only “Basic” and “Digest” authentication methods are supported.

<?php

   /* Redirect browser */
   header("Location: http://www.example.com/"); 

   /* Make sure that code below does not get executed when we redirect. */
   exit;
   
?>

The optional replace parameter indicates whether the header should replace a previous similar header, or add a second header of the same type, and response_code parameter forces the HTTP response code to the specified value.

To be able to force he client authentication, you need a .htaccess file in document root folder. Open a new text file, put the following text in it, and save it with .htaccess as its name.

CGIPassAuth On

Example

An example script fragment which would force client authentication on a page is as follows −

<?php
   if (!isset($_SERVER[''PHP_AUTH_USER''])) {
      header(''WWW-Authenticate: Basic realm="My Realm"'');
      header(''HTTP/1.0 401 Unauthorized'');
      echo ''User hits Cancel button'';7
      exit;
   } else {
      echo "<p>Hello {$_SERVER[''PHP_AUTH_USER'']}.</p>";
      echo "<p>You entered {$_SERVER[''PHP_AUTH_PW'']} as your password.</p>";
   }
?>

Output

When you visit the script in a browser, it pops up a dialog box as shown −

Once you click on the sign in button, there may be a backend script to authenticate the login credentials. Once authenticated, two server variables will be created with the keys PHP_AUTH_USER and PHP_AUTH_PW, which can be verified with the output of phpinfo() function.