Record deletion(I'm confused)

<?php
//Database connection details

$select_sql = 'Select  RBS_TFTPSERVERIPADDRESS, '.
		'RBS_TFTPFILENAME, RBS_ENROLMENT, RBS_CIPHERING, '.
'RBS_RFPI, RBS_IPADDRESS, RBS_MACADDRESS, RBS_SUBNETMASK, '.
'RBS_ROUTERIP from TBL_RBSNEW';
$result_id = ibase_query($select_sql) or die('Error retrieving records from TBL_RBSNEW.<br />Interbase Reported: '.ibase_errmsg());

?>
<b>To delete records</b></p>
<form name="editRBS" action="<?=$HTTP_SERVER_VARS['PHP_SELF']?>" method="POST">
<table cellpadding="0" cellspacing="0" width="909">
	
<?php
while ($row = ibase_fetch_object($result_id)) {
		$rfpi=$row->RBS_RFPI;
		$ciphering=$row->RBS_CIPHERING;
		$ipaddress=$row->RBS_IPADDRESS;
		$macaddress=$row->RBS_MACADDRESS;
		$subnetmask=$row->RBS_SUBNETMASK;
		$routerip=$row->RBS_ROUTERIP;
		$enrolment=$row->RBS_ENROLMENT;
		$tftpserver=$row->RBS_TFTPSERVERIPADDRESS;
		$tftpfile=$row->RBS_TFTPFILENAME;
		
	
// Add the delete flag.
$string = "update TBL_RBSNEW SET RBS_DELETE = 'Y'where RBS_ID = $id";

if ($delete==1) {
    $string .= "RBS_DELETE = 'Y'";
    }
// This line is always executed.
	$sql .= "where RBS_ID = $id";
	// This line does absolutely nothing.
	"where RBS_ID = $id";
   	$sql = "DELETE from TBL_RBSNEW where RBS_DELETE ='Y'"; 
    $result = ibase_query($sql);
    	
	
?>
	

<tr style="margin:0; padding:0;" align="left" valign="top">
<td width="156" height="23">
<input type="text" name="rfpi" value="<?=$rfpi?>">
</td>
<td width="154" height="23">
<input type="text" name="ip_address" value="<?=$ipaddress?>">
</td>
<td width="69" height="23">
<input type="checkbox" name="enrolment" value="1"<? if ($enrolment==1)echo "checked";?>>
</td>
<td width="65" height="23">
<input type="checkbox" name="ciphering" value="1" <? if ($ciphering==1)echo "checked";?>>
</td>
<td width="155" height="23">
<input type="text" name="mac_address" value="<?=$macaddress?>">
</td>
<td width="156" height="23">
<input type="text" name="subnet_mask" value="<?=$subnetmask?>">
</td>
<td width="156" height="23">
<input type="text" name="router_ip" value="<?=$routerip?>">
</td>
<td width="156" height="23">
<input type="text" name="tftp_server" value="<?=$tftpserver?>">
</td>
<td width="156" height="23">
<input type="text" name="tftp_file" value="<?=$tftpfile?>">
</td>
<td width="69" height="23">
<input type="checkbox" name="delete" value="<?=$row->RBS_ID?>">

</td>
</tr>
<?
}
?>
</table>
<input type="submit" name="submit" value="submit">
&nbsp;&nbsp;
</form>
</body>
</html>