<?php

$array1 = ["d8ab80f4f6"=>[2,3,5,7], '9087785727'=>[1,2,7,8]]; 
$array2 = ["d8ab80f4f6"=>[2,3], '9087785727'=>[1,2]]; 

$output = [];
foreach ($array1 as $key => $value) {
    $output[$key] = array_merge($value, $array2[$key]);
}
var_dump($output);