PHP5 recursive yield

<?php

$theArray = [
    'lorem' => [
        'ipsum' => [
            'dolor' => []
        ],
        'sit' => []
    ],
    'amet' => []
];

function expandArr($arr, $prefix = '') {
    foreach ($arr as $key=>$value) {
        echo '- yielding value ' . $prefix . $key . PHP_EOL;
        yield $prefix . $key;
        foreach (expandArr($value, $prefix . $key . '/') as $inner) {
            echo '- passing thru yield ' . $inner . PHP_EOL;
            yield $inner;
        }
    }
}

foreach (expandArr($theArray) as $str) {
    echo $str . PHP_EOL;
}