接收一个“；无效登录“；当尝试使用php和mysql登录时

我查看了许多网站，比较了各种登录脚本，但找不到任何解决方案。我输入了正确的信息，得到了一个"无效登录"，如果我不输入任何信息，我就会得到一个"非法登录"。我真的需要一些帮助来解决问题。这是代码

<?php 
  include_once ('dbc.php');
  $user_email = mysql_real_escape_string(isset($_POST['email']));
  if (isset($_POST['Submit'])=='Login')
  {
    $md5pass = md5($_POST['pwd']);
    $sql = "SELECT id,user_email FROM users WHERE 
              user_email = '$user_email' AND 
              user_pwd = '$md5pass' AND user_activated='1'"; 
    $result = mysql_query($sql) or die (mysql_error()); 
    $num = mysql_num_rows($result);
    if ( $num = 0 ) 
    { 
      // A matching row was found - the user is authenticated. 
      session_start(); 
      list($user_id,$user_email) = mysql_fetch_row($result);
      // this sets variables in the session 
      $_SESSION['user']= $user_email;  
      if (isset($_GET['ret']) && !empty($_GET['ret']))
      {
        header("Location: $_GET[ret]");
      } else {
        header("Location: myaccount.php");
      }
      //echo "Logged in...";
      exit();
    } 
    header("Location: login.php?msg=Invalid Login");
    //echo "Error:";
    exit;       
  }
?>

表单代码----

<td bgcolor="#e5ecf9" class="mnubody"><form name="form1" method="post" action="">
    <p>&nbsp;</p>
    <p align="center">Your Email: 
      <input name="email" type="text" id="email">
    </p>
    <p align="center"> Password: 
      <input name="pwd" type="password" id="pwd">
    </p>
    <p align="center"> 
      <input type="submit" name="Submit" value="Login">
    </p>
    <p align="center"><a href="register.php">Register</a> | <a href="forgot.php">Forgot</a></p>
  </form></td>