问如何在下拉选择菜单中添加验证？

EN

if(isset($_REQUEST['check'])){
if (isset($_POST['size']) && $_POST['size'] == '' ) {
echo 'Error, select category';
exit;
}}

<form action="" id="size1" name="size1" method="post"> 
<select style="background-color:#CCC" 
name="size" id="size"  onchange="this.form.submit();" >
<option value="">Select</option>
<option <?php if ($_POST['size'] == 'Small') 
print 'selected '; ?>  value="Small">Small</option>
<option <?php if ($_POST['size'] == 'Medium') 
print 'selected '; ?> value="Medium">Medium</option>
<option  <?php if ($_POST['size'] == 'Large') 
print 'selected '; ?> value="Large">Large</option>
<option   <?php if ($_POST['size'] == 'XL')
print 'selected '; ?> value="XL">XL</option>
</select>

<input type="button" class="button1" name="check" 
value="Add To Cart" onclick="addtocart(<?php echo $row3['id'];?>);" />
</form