код...
?>
		<input type="button" onclick="submitForm('страница_которую нужно_открыть.php')" value="Показать" >
		<script type="text/javascript"> 
			function submitForm(action) {
			document.forms['текущая_страница.php'].action = action;
   			document.forms['текущая_страница.php'].submit();
		}
		</script>
		<?
код...
		

<?php
include("include/header.php");
include("include/sidebar.php");
require_once('connectvars.php');
$dbc = mysqli_connect(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);
?>
<div id="content">
    <form id="form1" method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <?php
    ...
		?>
		<input type="button" onclick="submitForm('page_list.php')" value="Показать" >
		
		<script type="text/javascript"> 

			function submitForm(action) {
			document.forms['form1'].action = action;
   			document.forms['form1'].submit();
		}

		</script>
		<?
		
    }
        else {echo 'Ошибка ввода данных';}
    }
    ?>

</form>
</div>

<?php
mysqli_close($dbc);
include("include/footer.php");?>