php mysql 用户登录源代码

源代码如下： 

<?php
   function authenticate_user() {
      header(\'WWW-Authenticate: Basic realm=\"Secret Stash\"\');
      header(\"HTTP/1.0 401 Unauthorized\");
      exit;
   }

   if (! isset($_SERVER[\'PHP_AUTH_USER\'])) {
      authenticate_user();
   } else {
      mysql_pconnect(\"localhost\",\"authenticator\",\"secret\") or die(\"Can\'t connect to data  server!\");
      mysql_select_db(\"db\") or die(\"Can\'t select authentication data !\");

      $query = \"SELECT username, pswd FROM user WHERE username=\'$_SERVER[PHP_AUTH_USER]\' AND pswd=MD5(\'$_SERVER[PHP_AUTH_PW]\')\";

      $result = mysql_query($query);

      // If nothing was found, reprompt the user for the login information.
      if (mysql_num_rows($result) == 0) {
         authenticate_user();
      }
   }
?>