How can I count element of xml from a recursive function in PHP?

GetNavigations($child, $level+1, $mrg+1) will pass the same value of $mrg to all children of a node, however many children it has, because $mrg is not being changed anywhere else inside that loop. Instead of $mrg+1, you could pass ++$mrg - or more readably, add $mrg++; as the line before and just pass $mrg.

However, you will still have the problem that the function only knows how many direct children have been displayed, not how many descendants - if you call GetNavigations with an $mrg value of 2, and it displays 20 nested items, your next value of $mrg will be 3, not 23! Although they all have the same name, each time you run the function, you have a new $mrg variable.

To get around that, you can either:

• Pass $mrg in by reference (by changing the function declaration to be function GetNavigations(SimpleXMLElement $element, $level = 0, &$mrg) with the added &, so that all copies of the function can write to the same variable.
• Pass the new value of $mrg out as the return value of the function.

I would probably prefer the second approach, as it's a bit clearer to anyone reading the code what's going on:

function GetNavigations(SimpleXMLElement $element, $level = 0, $mrg = 0)
{   
    /* [snip] */

    if($element->getName() == 'GroupName')
    {
            // Increment counter, because we're displaying something
            $mrg++;

            /* [snip] */
    }

    /* [snip] */

    if(count($children))
    {
        foreach($children as $child)
        {
            // Recurse, and get incremented value of counter
            $mrg = GetNavigations($child, $level+1, $mrg);
        } 
    }

    /* [snip] */

    // Let caller know where the counter has got to
    return $mrg;
}