<?php
$ar1 = ['a' => ['b' => 'c', 'd' => 'e']];
$ar2 = ['a' => ['d' => 'f']];

function merge($from, $to)
{
    $result = $to;

    foreach ($from as $itemKey => $item) {
        foreach($item as $k => $v) {
            $result[$itemKey][$k] = $v;
        }
    }

    return $result;
}


echo merge($ar2, $ar1) === ['a' => ['b' => 'c', 'd' => 'f']];