PHP MySQL登录表单不起作用

我正在尝试让用户登录。我已经制作了注册表格，数据库已经正确连接。它不断跳过第一个IF语句，直接进入"出现错误"。有人知道为什么它不起作用吗？

<?php  
$pageTitle = "Sign In";
$pageCategory = "Sign In";
$pageCategoryurl = "/signin.php";

//signup.php   
include($_SERVER["DOCUMENT_ROOT"] . "/inc/header.php"); 
include($_SERVER["DOCUMENT_ROOT"] . "/inc/search.php");
?>
<div class="content">
<div id="signinheader"><h2>Sign in</h2></div><div style="clear:both"></div> 
<?php
if(isset($_SESSION['signed_in']) && $_SESSION['signed_in'] == true)  
{  
    echo 'You are already signed in, you can <a href="signout.php">sign out</a> if you     want.';  
}
else
{  
    if($_SERVER['REQUEST_METHOD'] != 'POST')  
    {  
        /*the form hasn't been posted yet, display it 
          note that the action="" will cause the form to post to the same page it is on */  
        echo '<form method="post" action="">  
            <table>
            <tr>
                <th><label for="username" class="signinlabel">Username:</label></th>
                <td><input type="text" name="username" class="signininput"></td>
            </tr>
            <tr>  
            <th><label for="userpass" class="signinlabel">Password:</label></th>
                <td><input type="password" name="userpass" class="signininput"></td>
            </tr> 
            </table>
            <input type="submit" value="Sign In" class="signinbutton">  
         </form>'; 
    } 
    else 
    { 
       /* so, the form has been posted, we'll process the data in three steps:  
            1.  Check the data  
            2.  Let the user refill the wrong fields (if necessary)  
            3.  Save the data   
       */   
       $errors = array(); /* declare the array for later use */  

       if(!isset($_POST['username']) OR empty($_POST['username']))  
      {  
         $errors[] = 'The username field must not be empty.';  
      }  
       if(!isset($_POST['userpass']) OR empty($_POST['userpass']))  
      {  
     $errors[] = 'The password field must not be empty.';  
  }   
   if(!empty($errors)) /*check for an empty array, if there are errors, they're in this array (note the ! operator)*/  
   {  
        echo '<div id="signinerror"><h3>Uh-oh.. a couple of fields are not filled in correctly..</h3>'; 
        echo '<ul>'; 
        foreach($errors as $key => $value) /* walk through the array so all the errors get displayed */ 
        { 
            echo '<li class="signinerrorli">' . $value . '</li>'; /* this generates a nice error list */ 
        } 
        echo '</ul></div><div style="clear:both"></div>'; 
   } 
   else 
   { 
      //the form has been posted without, so save it 
      //notice the use of mysql_real_escape_string, keep everything safe! 
      //also notice the sha1 function which hashes the password 
      $username = $_POST['username'];
     $userpass = sha1($_POST['userpass']);
      $result = mysqli_query($con,"SELECT * FROM users 
        WHERE username = '$username' AND userpass = '$userpass");

      if(!$result)  
      {  
            //something went wrong, display the error  
            echo 'Something went wrong while signing in. Please try again later.'; 
            //echo mysqli_error(); //debugging purposes, uncomment when needed 
      } 
      else 
     { 
        //the query was successfully executed, there are 2 possibilities 
        //1. the query returned data, the user can be signed in 
        //2. the query returned an empty result set, the credentials were wrong 
        if(mysqli_num_rows($result) == 0) 
        { 
                echo 'You have supplied a wrong user/password combination. Please try again.'; 
        } 
        else 
        { 
           $_SESSION['signed_in'] = true; 
           //we also put the user_id and user_name values in the $_SESSION, so we can use it at various pages 
           while($row = mysqli_fetch_assoc($result)) 
           { 
              $_SESSION['user_id']   = $row['user_id']; 
              $_SESSION['username']  = $row['username']; 
              $_SESSION['useremail'] = $row['useremail']; 
           } 
           echo 'Welcome, ' . $_SESSION['username'] . '. <a href="/home.php">Proceed to the homepage</a>.'; 
             } 
          }   
   } 
} 
}
?>
</div> 
<?php
include($_SERVER["DOCUMENT_ROOT"] . "/inc/footer.php");
?>