<?php
 
$tree = array(
    0   =>  100,
    1   =>  200,
    2   =>  array(
        0   =>  7,
        1   =>  8,
        2   =>  9        
    )
);
 
function walkAroundTheTree(array &$branch)
{
    foreach ($branch as &$node) {
        if (is_array($node)) {
            // Если у нас вложенный массив
            walkAroundTheTree($node);
        } else {
            // Если число
            $node = $node * 2;
        }
    }
}
 
var_dump($tree);
walkAroundTheTree($tree);
var_dump($tree);

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