Warning: in_array() [function.in-array]: Wrong datatype for second argument in C:\AppServ\www\library\templates\nav_right.tpl.php on line 62

Hello Experts,
   I try to code php program for recently view items. and it can show the last items that user watched, but when i load this page the first time it will show error message as followed:

Warning: in_array() [function.in-array]: Wrong datatype for second argument in C:\AppServ\www\library\templates\nav_right.tpl.php on line 62

I attached code. Please help me... Many thanks in advance.

<?
$gal_item_id=$_GET['gal_item_id'];
 
// insert codes to retrive title from gal_item_id
		$conn_mysql->db_connect(DB_SERVER_HOST, DB_SERVER_USERNAME, DB_SERVER_PASSWORD, DB_SERVER_DATABASE);
		# Mode SQL Command : Select data in condition
		$sql = " SELECT * from gallery_item where gal_item_id='$gal_item_id' ";
 
		$res['select_item'] = $conn_mysql->query($sql);		
		# Loop data in to variable : Fetch a result row as an associative array
		$arr['detail_item'] = $conn_mysql->fetch_row($res['select_item']);
			# Define variable for use easy
			$gal_item_title = $arr['detail_item']['gal_item_title'];
 
 
?>
 
<?
   $item_link="<a href='?module=gallery&action=info_item&gal_item_id=$gal_item_id'>$gal_item_title</a>";
 
if (! in_array($item_link, $_SESSION['articles']))
$_SESSION['articles'][] = "$item_link";
// Output list of requested articles
 
 
 
 
echo "<ul>";
foreach($_SESSION['articles'] as $doc) echo "<li>$doc</li>";
//echo "<p>$gal_item_title</p></p>";
echo "</ul>";
 
 
 
?>

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