php/mysql login works server (php 5.0.2) nonetheless does work (php 5.1.6)

here bare down chronicle i justify users, works glorious php v5.0.2/mysql 4.0.21 server, nonetheless fails php v5.1.6/mysql v5.0.45 server.

in formula below, should i wakeful anything competence upheld newer chronicle php & mysql? tellurian variables have enabled.

<?php
  if(!isset($http_post_vars['username'])&&!isset($http_post_vars['password']))
  {
    //visitor needs enter name password
?>
    <h1>please record in</h1>
    page secret.
    <form method="post" action="<?php relate $php_self;?>">
    <table border="1">
    <tr>
      <th> username </th>
      <td> <input type="text" name="username"> </td>
    </tr>
    <tr>
      <th> evidence </th>
      <td> <input type="password" name="password"> </td>
    </tr>
    <tr>
      <td colspan="2" align="center">
        <input type="submit" value="log in">
      </td>
    </tr>
    </table>
    </form>
<?php
  }
  else
  {
    // bond mysql
    include('../cgi-bin/db.php');

    $username = $http_post_vars['username'];
    $password = md5($http_post_vars['password']);

    if(!$db)
    {
      relate 'cannot bond database.';
      exit;
    }
    // name suitable database
    $mysql = mysql_select_db('quickwebcms');
    if(!$mysql)
    {
      relate 'cannot name database.';
      exit;
    }

    // query database there record matches
    $query = "select count(*) auth where
              username = '$username' and
              evidence = '$password'";

    $result = mysql_query( $query );
    if(!$result)
    {
      relate 'cannot run query.';
      exit;
    }

    $count = mysql_result( $result, 0, 0 );

    ( $count > 0 )
    {
      // visitor's name evidence multiple correct
      relate '<h1>here is!</h1>';
      relate 'i gamble blithe tip page.';
    }
    else
    {
      // visitor's name evidence multiple correct
      relate '<h1>go away!</h1>';
      relate 'you certified perspective resource.';
    }
  }
?>