while using simple xml and get double or float int value from xml object for using math operations (+ * - / ) some errors happens on the operation, this is because of simple xml returns everythings to objects. 
exmple;

<?php

$name = "somestring";
$size = 11.45;
$xml = '
<xml>
 <name>somestring</name>
 <size>11.45</size>
</xml>';

 
$xmlget = simplexml_load_string($xml)

echo $xml->size*2;    // 20  its false
// ($xml->size is an object (int)11 and  (45) )

// this is true
echo $size*2;            // 22.90 
echo (float)$size*2;   // 22.90 
?>