Are you looking for array index variables like this?
<?php
$xml = new SimpleXMLElement($row['feed_result']);
$a = 'Sensor';
$aindex = 0; // <------
$b = 'Condition';
$bindex = 6; // <------
$c = 'LastReading';
echo 'Sensor 1 : '.$xml->{$a}[$aindex]->{$b}[$bindex]->{$c}.'<br />';
?>