how do i add validation on drop down select menu?

[magic]

how do i add validation on drop down select menu i have code and i'm working on but problem is that is not working nothing display error message. I want to be display error message if drop down selection values shouldn't be selected before click on add to cart button?\[code\]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="http://stackoverflow.com/questions/15848878/Small">Small</option><option <?php if ($_POST['size'] == 'Medium') print 'selected '; ?> value="http://stackoverflow.com/questions/15848878/Medium">Medium</option><option <?php if ($_POST['size'] == 'Large') print 'selected '; ?> value="http://stackoverflow.com/questions/15848878/Large">Large</option><option <?php if ($_POST['size'] == 'XL')print 'selected '; ?> value="http://stackoverflow.com/questions/15848878/XL">XL</option></select><input type="button" class="button1" name="check" value="http://stackoverflow.com/questions/15848878/Add To Cart" onclick="addtocart(<?php echo $row3['id'];?>);" /></form\[/code\]