It is possible to use the header() function to send an “Authentication Required” message to the client browser causing it to pop up a Username/Password input window. 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. Both “Basic” and “Digest” (since PHP 5.1.0) authentication methods are supported

Basic HTTP Authentication

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

Digest HTTP Authentication

<?php
 $realm = 'Restricted area';
 $users = array('admin' => 'mypass', 'guest' => 'guest');

if (empty($_SERVER['PHP_AUTH_DIGEST'])) {
 header('HTTP/1.1 401 Unauthorized');
 header('WWW-Authenticate: Digest realm="'.$realm.'",qop="auth",nonce="'.uniqid().'",opaque="'.md5($realm).'"');

die('Text to send if user hits Cancel button');
 }

if (!($data = http_digest_parse($_SERVER['PHP_AUTH_DIGEST'])) ||
 !isset($users[$data['username']]))
 die('Wrong Credentials!');

$A1 = md5($data['username'] . ':' . $realm . ':' . $users[$data['username']]);
 $A2 = md5($_SERVER['REQUEST_METHOD'].':'.$data['uri']);
 $valid_response = md5($A1.':'.$data['nonce'].':'.$data['nc'].':'.$data['cnonce'].':'.$data['qop'].':'.$A2);

if ($data['response'] != $valid_response)
 die('Wrong Credentials!');

echo 'You are logged in as: ' . $data['username'];

function http_digest_parse($txt)
 {
 $needed_parts = array('nonce'=>1, 'nc'=>1, 'cnonce'=>1, 'qop'=>1, 'username'=>1, 'uri'=>1, 'response'=>1);
 $data = array();
 $keys = implode('|', array_keys($needed_parts));

preg_match_all('@(' . $keys . ')=(?:([\'"])([^\2]+?)\2|([^\s,]+))@', $txt, $matches, PREG_SET_ORDER);

foreach ($matches as $m) {
 $data[$m[1]] = $m[3] ? $m[3] : $m[4];
 unset($needed_parts[$m[1]]);
 }

return $needed_parts ? false : $data;
 }
 ?>