选项选择，检查选项是否选中？

Question

I had search internet, most tutorial show how to if $what= what, echo 'selected', but I want to do something opposite, if this option selected, echo $what = what,look at my code below. I don't want to touch the javascript, is that possible to do it?

<?php
<select id="post_show_per_page">
     <option value="30">30</option>
     <option value="60" >60</option>
     <option value="90" >90</option>
     <option value="120" >120</option> 
   </select>
// check if #post_show_per_page value is 30, $post_show_per_page = 30;
// check if #post_show_per_page value is 60, $post_show_per_page = 60;

?>

</div>