array_walk_recursive.php

<?php
$arr = [];
for ($i = 0; $i < 5000; $i++) {
    $arr[$i] = range($i, $i+1);
}

$res = array();
array_walk_recursive($arr, function ($item, $key) use (&$res) {
    $res[] = $item;
});

foreach.php

<?php
$arr = [];
for ($i = 0; $i < 5000; $i++) {
    $arr[$i] = range($i, $i+1);
}

$res = array();
foreach ($arr as $item) {
    foreach ($item as $value) {
        $res[] = $value;
    }
}