Set form action to $_SERVER['PHP_SELF'] like below: $_SERVER['PHP_SELF'] is a PHP super global variable which returns the filename of the currently executing script.

   <form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <select name="category" >
        <option value="nocategory">No Category</option>
        <option value="toyota">Toyota</option>
        <option value="nissan">Nissan</option>
        <option value="mazda">Mazda</option>
        <option value="volvo">Volvo</option>
     </select>

    <input name="submit"  value="Search" type="submit">

   </form> 


    <?php

       if (isset($_POST['submit'])){
       $category = $_POST['category'];
       echo $category;
       }

    ?>