comparing security question values

on my login page i ask the member to enter there username,password and answer a simple security question.

but each time i keep getting the error page...


$numbers = array();
      
      $numbers[1] = 'one';
      $numbers[2] = 'two';
      $numbers[3] = 'three';
      $numbers[4] = 'four';
      $numbers[5] = 'five';
      
      $first = array_rand($numbers);
      $second = array_rand($numbers);
      
<form name="loginFrm" method="post" action="'.$_SERVER['PHP_SELF'].'" >
      <input type="hidden" value="'.time().'" name="currenttime">'."\n";
      <input type="hidden" value="'.md5($first+$second+time()).'" name="sum">'."\n";
      Username:<input type="text" name="username">
      Password:<input type="password" name="password">
Security Question:What is '.$numbers[$first].' plus '.$numbers[$second].'? <input type="text" name="answer" size="2" value="">
<input type="submit" value="Login">
</form>


$s_Username .= trim($_POST['username']);
$s_Password .= trim($_POST['password']);
      
$s_Answer .= trim($_POST['answer']);
$s_CurrentTime .= trim($_POST['currenttime']);
      
$s_SumAnswer = md5($s_Answer+$s_CurrentTime);
$s_Sum .= trim($_POST['sum']);
      
if($s_Sum == $s_SumAnswer)
{      
                  //Problems with answer to security question
}
else
{
    // check username and password
    if ok
        login them in
    else
        ///problems
}