unset variable..

[Codeman0013]

I want to use

unset($_POST['productCat_id']);

to clear out my url if the user selects a new industry_id . Here is my code maybe you can help me understand where it goes...

<form name="distributers" />			  <table class="setTbl listTbl" width="90%" cellpadding="0" cellspacing="0" border="0">				<tr><td width="15%">Industry:</td>				<td width="85%">				<select name="industry_id" id="industry" onchange="document.distributers.submit();">								  <option value="">Select an Industry...</option>				  <?php do { ?>				 <?php   					 print "<option value=\"{$row_rs_industry['industry_id']}\"" .					(isset($_GET['industry_id']) && $_GET['industry_id'] == $row_rs_industry['industry_id'] ? " selected>" : ">") .    					 "{$row_rs_industry['industry_name']}</option>\n";?>						 						  				  <?php } while ($row_rs_industry = mysql_fetch_assoc($rs_industry)); ?>				</select> </td></tr>				 <tr><td>Product category:</td><td>				 				 <?php if ($_GET['industry_id'] == null || $_GET['industry_id'] == "")  { ?>				 				 <select name="productCat_id" id="productcat" onchange="document.distributers.submit();" disabled>;				 <?php } else { 					mysql_select_db($database_conn_dj, $conn_dj);					$query_rs_productcategory = "SELECT productCat_id, productCat_name, industry_id FROM tbl_productcat WHERE industry_id = '$_GET[industry_id]' ORDER BY productCat_name";					$rs_productcategory = mysql_query($query_rs_productcategory, $conn_dj) or die(mysql_error());					$row_rs_productcategory = mysql_fetch_assoc($rs_productcategory);					$totalRows_rs_productcategory = mysql_num_rows($rs_productcategory);				 ?>			 			<select name="productCat_id" id="productcat" onchange="document.distributers.submit();" >				  <option value="">Select a product category...</option>				<?php do {   					 print "<option value=\"{$row_rs_productcategory['productCat_id']}\"" .					(isset($_GET['productCat_id']) && $_GET['productCat_id'] == $row_rs_productcategory['productCat_id'] ? " selected>" : ">") .    					 "{$row_rs_productcategory['productCat_name']}</option>\n";				 } while ($row_rs_productcategory = mysql_fetch_assoc($rs_productcategory)); ?>				</select>		 		<?php } ?></td>								<tr><td>Product:</td><td>									 <?php if ($_GET['productCat_id'] == null || $_GET['productCat_id'] == "" || ($_GET['industry_id'] == null || $_GET['industry_id'] == "")) { ?>				 <select name="products_id" id="products" onchange="document.distributers.submit();" disabled>;				 <?php } else { 						mysql_select_db($database_conn_dj, $conn_dj);					$query_rs_product = "SELECT products_id, products_name, productCat_id FROM tbl_products WHERE productCat_id = '$_GET[productCat_id]' ORDER BY products_name";					$rs_product = mysql_query($query_rs_product, $conn_dj) or die(mysql_error());					$row_rs_product = mysql_fetch_assoc($rs_product);					$totalRows_rs_product = mysql_num_rows($rs_product);					?>		 			<select name="products_id" id="products" onchange="document.distributers.submit();" >				  <option value="">Select a product...</option>					<?php do {	   					 print "<option value=\"{$row_rs_product['products_id']}\"" .						(isset($_GET['products_id']) && $_GET['products_id'] == $row_rs_product['products_id'] ? " selected>" : ">") . 	   					 "{$row_rs_product['products_name']}</option>\n";					 } while ($row_rs_product = mysql_fetch_assoc($rs_product)); ?>					</select>				<?php } ?> 									</td></tr>				<tr><td> </td><td><input type="submit" value="Find" name="find"></td></tr>		  </table>			</form>