How do you put content in xml file using file_put_contents in sub-child element

Question

I know how to put content to child element I google and can't find solution to put content to sub-child element. I have following structure in my xml file

<pagevalues>
    <ac>
        <acb1/>
        <acb2/>
        <acb3/>
        <acb4/>
        <acb5/>
        <acpc/>
    </ac>
........
.....
</pagevalues>

this is the code to put value in xml file

$xml->$ac = $actotal;
file_put_contents("$url", $xml->asXML());

Answer 1

I got it

$xml->ac->acb1 = $actotal; file_put_contents("$url", $xml->asXML());