Output only keys of nested array as string.

Question

I have a function that generates a multidimensional to represent a tree structure. A print_r of what it returns looks something like this:

Array (
    [A] => Array (
        [A1] =>
        [A2] =>
    )
    [B] => Array (
        [B1] => Array (
            [B1a] =>
        )
    )
)

What I need is a recursive function that output this in depth first order, like this:

A
  A1
  A2
B
  B1
    B1a

or just

A, A1, A2, B, B1, B1a

I have tried to solve this for hours now so please help me.

Answer 1

use recursive function eg:

<?php
$arr = array(
    'A'=>array(
        "A1"=>1,
        "A2"=>1,
        ),
        "B"=>array(
            "B1"=>array(
                "B2a"=>1,
                )
            ),
    );

function print_key($arr, $depth = 0){
    $pre_str = str_repeat("\t", $depth);
    foreach($arr as $k=>$v){
        echo $pre_str.$k."\n";
        if(is_array($v))
            print_key($v, $depth+1);
    }
}
print_key($arr);

Answer 2

You don't need to recursively scan the array, since print_r already does it for you. You could just call print_r and then parse the output:

preg_match_all('/^[^\[]*\[([^\]]*)\][^\]]*$/m', print_r($arr, true), $matches);
$result = implode(', ', $matches[1]);

echo $result; // A, A1, A2, B, B1, B1a

Demo

Answer 3

I realized I can just try to mimic the print_r function so I found a useable function here: php print_r nice table