PHP program to create new xml from old one?

I have some xml files with following values in it.

<RxCount CorpNumber="238" refillInternal="55" newInternal="26"/>
<RxCount CorpNumber="449" refillInternal="125" newInternal="79"/>
<RxCount CorpNumber="441" refillInternal="142" newInternal="215"/>
<RxCount CorpNumber="427" refillInternal="122" newInternal="186"/>
<RxCount CorpNumber="207" refillInternal="133" newInternal="99"/>

What i need to do is i need to create a new xml sheet where the value of refillInternal is equal to the value of newInternal+refillInternal.

for example if we take the below row...

<RxCount CorpNumber="238" refillInternal="55" newInternal="26"/>

the new row should be like

<RxCount CorpNumber="238" refillInternal="81" newInternal="26"/>

is there is any way to complete this. i have more than 10 xml docs which containing more than 200 rows.