keeping check box checked after submiting the form

i have problem guys to keep the checkbox checked after submiting the form , i tried this code but its not working can you please some body hep me and tell me whats exactly wrong with it ,plus i do believe there must be an array created but not really sure howand where to set the array in my file.

<P>
         <label for ='product'>product</label>
            milk<input type='checkbox' name='optional[]' value='milk' id='milk'/>
            <?php
            if(isset($_POST['optional']) && in_array('milk',$_POST['optional'])) echo "checked='checked'";
            ?> 
            butter<input type='checkbox' name='optional[]' value='butter' id='butter'/> 
            <?php
            if(isset($_POST['optional']) && in_array('butter',$_POST['optional'])) echo "checked='checked'";
            ?> 
            cheese<input type='checkbox' name='optional[]' value='cheese' id='cheese'/>
            <?php
            if(isset($_POST['optional']) && in_array('cheese',$_POST['optional'])) echo "checked='checked'";
            ?>