Why is my $_GET variable coming up undefined??

Hi, I set 'clear' in my link:

<a href="logfile.php?clear=true">Clear Log File</a>

I try to get it:

if($_GET['clear'] == 'true') {
      file_put_contents($logfile, '');
      // Add the first log entry
      log_action('Logs Cleared', "by User Id {$session->user_id}");
      // redirect to this same pages so that the URL won't have "clear=true" anymore
      ob_end_clean();
      redirect_to('logfile.php');
}

But for some reason thats not working??  Anyone know why?

Notice: Undefined index: clear in /hermes/web09c/b2950/moo.auroriellacom/logfile.php on line 11

<?php
ini_set('display_errors' ,1); 
error_reporting(E_ALL);

require_once 'includes/library.php';
$title = "Auroriella Management";
include 'header.php';

$logfile = 'logs/logs.txt';

if($_GET['clear'] == 'true') {
	file_put_contents($logfile, '');
	// Add the first log entry
	log_action('Logs Cleared', "by User Id {$session->user_id}");
	// redirect to this same pages so that the URL won't have "clear=true" anymore
	ob_end_clean();
	redirect_to('logfile.php');
}

//validate_fields(); // Validation
find_selected_id(); // Needs to go below process_fields to show updates
echo nav($select_nav);

include 'sidebar.php';
echo menu($select_menu, $select_product, $select_metal);

include 'display.php';
//user_info();
?>
<a href="logfile.php?clear=true">Clear Log File</a>
<?php
echo "<p>";
if(file_exists($logfile) && is_readable($logfile) && $handle = fopen($logfile, 'r')) {
	echo "<ul class=\"logs\">";
	while(!feof($handle)) { // Until we get to the end of the file
		$entry = fgets($handle); // fgets works bc we know there are new lines
		if(trim($entry) != "") { // Trim white space and make sure its not nothing
			echo "<li>{$entry}</li>";
		}
	}
	echo "</ul>";
	fclose($handle);
}
else {
	echo "Could not read from {$logfile}.";
}
echo "<p>";

require 'footer.php';
?>

Select allOpen in new window