<?php
       $arr_book = array("1" => "1 смена", "2" => "2 смена"); //Тут массив  
    ?>
    <form id="newFormAdv" action="#" method="post">
        <table>
    <select name="book" onchange="document.getElementById('newFormAdv').submit()">    
        <?php
        foreach($arr_book as $value => $book) {
         $select = isset($_POST['book']) && $value == $_POST['book']?'selected>':'>';
         echo '<option value="'.$value.'"'.$select.$book.'</option>';
        }
        ?>
    </select></td></tr>
    <?php if(isset($_POST['book']) && $_POST['book'] > 0) { ?>
            <tr><td><p>Выберите категорию</p>
          <?php
            $smen = $_POST['book'];
            echo $smen;
        ?>

        </table>
    </form>

 if ($todaytime > '08.00.00' and $todaytime < '20.00.00')
         $acti= 2;
        else  $acti = 1;

if ($todaytime > '08.00.00' and $todaytime < '20.00.00')
		 $arr_book = array( "2" => "2 смена", "1" => "1 смена"); 				
			else 			
			 $arr_book = array(  "1" => "1 смена", "2" => "2 смена");