檢查，以確保一個選項已從<select></select>列表

Question

選擇我有我的一切形式的工作，除了要驗證從選擇菜單中的選項已經chosen..it顯示錯誤，但一旦選擇一些犯規走開

在html

<div id="budget_input"> 
       <label for="budget">Budget</label> 
      <select name="mydropdown"> 
      <option value="" id="mydropdown"> -- choose one --</option> 
      <option value="firstchoice">$0 - $1,000</option> 
      <option value="secondchoice">$1,000 - $2,000</option> 
      <option value="thirdchoice">$3,000 +</option> 
      </select> 
</div> 

PHP

$name = trim(stripslashes(htmlspecialchars($_POST['name'])));   
$email = trim(stripslashes(htmlspecialchars($_POST['email']))); 
$mydropdown = trim(stripslashes(htmlspecialchars($_POST['mydropdown']))); 

$error_message = '';  
    $reg_exp = "/^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9-]+\.[a-zA-Z.]{2,4}$/"; 

    if (!preg_match($reg_exp, $email)) { 

       $error_message .= "<p>A valid email address is required.</p>";    
    } 
    if (empty($name)) { 

       $error_message .= "<p>Please provide your name.</p>";    
    } 

    if (empty($mydropdown)) { 

       $error_message .= "<p>Please select an item from the list.</p>"; 
    } 

JS部分示出了用於在客戶端

var valid = ''; 
var required = ' is required.'; 
var name = $('form #name').val(); 
var email = $('form #email').val(); 
var mydropdown = $('form #mydropdown').val(); 

if (name == '' || name.length <= 2) { 
     valid = '<p>Your name' + required +'</p>'; 
    } 

    if (!email.match(/^([a-z0-9._-]+@[a-z0-9._-]+\.[a-z]{2,4}$)/i)) { 
     valid += '<p>Your email' + required +'</p>';             
    } 

    if (mydropdown == '') { 
     valid += '<p>An item from the list' + required +'</p>'; 

    }