Can anyone tell me what i'am doing wrong..

Code PHP:
<?php
session_start();
include_once("conn.php");
$cat=mssql_query("Select * from tblCat");
 
 
?>
<html>
<head>
</head>
<body>
<table>
 
		<?php
		echo '<tr>';
			echo '<td>Category</td>';
			echo '<td>';
 
 
					echo "<select name='catname' >";
					echo "<option></option>";
					while($catname=mssql_fetch_assoc($cat)){
						echo "<option value='".$catname[CatName]."'>".$catname['CatName']."</option>";
					}
 
			echo '</td>';
		echo '</tr>';
		echo '<tr>';
			echo '<td>Sub-Category</td>';
			echo '<td>';
				if($subcat=mssql_query("Select * from tblSubCat where CatName='$_POST[catname]'"))
				echo "<Select name='catname'>";
				echo "<option></option>";
				while($subcatname=mssql_fetch_assoc($subcat)){
					echo "<option value='".$subcatname[CatName]."'>".$subcatname['CatName']."</option>";
				}
		echo '</tr>';	
		?>
 
 
</table>
</body>
</html>