ho provato in questo modo

codice:
<?php
include "init.php";
include_once('tbs_class.php');
$TBS = new clsTinyButStrong;
?>
<html>
	<head>
		<title>B2BMOMO</title>
		<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" integrity="sha384-1q8mTJOASx8j1Au+a5WDVnPi2lkFfwwEAa8hDDdjZlpLegxhjVME1fgjWPGmkzs7" crossorigin="anonymous">
		<link rel="stylesheet" href="/include/css/bootflat.min.css">
		<link rel="stylesheet" href="/include/css/app.css">
		<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/s/bs/pdfmake-0.1.18,dt-1.10.10,af-2.1.0,b-1.1.0,b-colvis-1.1.0,b-html5-1.1.0,b-print-1.1.0,cr-1.3.0,fc-3.2.0,fh-3.1.0,kt-2.1.0,r-2.0.0,rr-1.1.0,sc-1.4.0,se-1.1.0/datatables.min.css"/>
	</head>
	<body>
	<div align="center">ELENCA TABELLE PRESENTI NEL DB</div>
		<form method="post" action="index.php" style="text-align: center;">
			<INPUT type="submit" value="Invia">
			<br><br>
		</form>
		<?php
		if ( isset($_POST) )
		{
		$query = "SHOW tables FROM $db_name"; 
		if (mysql_query($query)) { 
		echo "Lista delle tabelle nel db"; 
		} else { 
		echo "Errore: ". mysql_error(); }
		} 
		?>
			<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/2.1.4/jquery.js"></script>
		<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js" integrity="sha384-0mSbJDEHialfmuBBQP6A4Qrprq5OVfW37PRR3j5ELqxss1yVqOtnepnHVP9aJ7xS" crossorigin="anonymous"></script>
		<script src="/include/js/bootstrap-contextmenu.js"></script>
		<script type="text/javascript" src="/include/js/bootflat.min.js"></script>
		<script type="text/javascript" src="https://cdn.datatables.net/s/bs/pdfmake-0.1.18,dt-1.10.10,af-2.1.0,b-1.1.0,b-colvis-1.1.0,b-html5-1.1.0,b-print-1.1.0,cr-1.3.0,fc-3.2.0,fh-3.1.0,kt-2.1.0,r-2.0.0,rr-1.1.0,sc-1.4.0,se-1.1.0/datatables.min.js"></script>
	</body>
</html>
mi genera il risultato prima che premo il bottone e in piu mi da errore del server 500, come posso fare perchè me lo generi successivamente