Function with foreach unexpectedly retaining variable's value - php

Question

I'm trying to grab and print all paths from root to leaf in a nested PHP array. So for example, if this is my array:
$tree = array(
"A" => "w",
"B" => array("x" => array("y", "z"))
)
I want the following as output:
Array
(
[0] => Aw
[1] => Bxy
[2] => Bxz
)
Here's my function at present:
function traverse($input, $myPath) {
global $allPaths;
if(is_array($input)) {
foreach($input as $k => $v) {
if(!is_int($k)) $myPath .= $k;
traverse($v, $myPath);
}
} else {
$myPath .= $input;
$allPaths[] = $myPath;
}
}
When I run this code:
$allPaths = array();
echo "<pre>";
traverse($tree, "");
print_r($allPaths);
echo "</pre>";
The output is this:
Array
(
[0] => Aw
[1] => ABxy
[2] => ABxz
)
That's almost correct, but for some reason the A is being retained when it reaches the "B" part instead of being reset as I would expect.
I've re- and re-read the code and tried every debugging message I can think of, and still don't get what's going on. I'm sure it's either something basic (perhaps so much so that if I haven't seen it yet, I never will) or I'm just not getting how variable scope is working here.

Answer 1

The 1st call to the function runs over the top level array, so the top level keys always get appended.
To fix, pass a copy of $myPath with $k appened to the recursive calls, rather than appending to $myPath directly:
function traverse($input, $myPath) {
global $allPaths;
if(is_array($input)) {
foreach($input as $k => $v) {
if(!is_int($k)){
traverse($v, $myPath . $k);
}else{
traverse($v, $myPath);
}
}
} else {
$myPath .= $input;
$allPaths[] = $myPath;
}
}