Mine

if ((isset($_POST['submit'])==true) && (isset($_POST['checkbox'])==true)) 
  		{ header("location: http://www.fullurl.com/nextpage.php"); }  
  	else if ((isset($_POST['submit'])==true) && (isset($_POST['checkbox'])==false))
  	{ $Error = "You must check the box to continue."; }
  ?>
  <html>
  <head>  <body>
  	<form action="test1.php" method="post"> 
  		<? if (isset($Error)==true) { echo $Error; } ?> 
  		<input type="checkbox" name="checkbox"> You must check this. 
  		<input type="submit" name="submit">
  	</form>
 </body>