.htaccess

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule ^([^?]*)$ /secure.php?path=$1 [E=HTTP_AUTHORIZATION:%{HTTP:Authorization},L]
</IfModule>

secure.php

<?php

if( !isset( $_SERVER[ 'HTTP_AUTHORIZATION'] ) && isset( $_SERVER[ 'REDIRECT_HTTP_AUTHORIZATION' ] ) ) {
  $_SERVER[ 'HTTP_AUTHORIZATION' ] = $_SERVER[ 'REDIRECT_HTTP_AUTHORIZATION' ];
}

list($_SERVER['PHP_AUTH_USER'], $_SERVER['PHP_AUTH_PW']) = explode(':', base64_decode(substr($_SERVER['HTTP_AUTHORIZATION'], 6)));

header( 'cache-control: no-cache' );

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