<?php

$array = [
    ['ABC' => 1],
    ['ADC' => 2],
    ['AXC' => 15],
    ['XAC' => 15]
];

// primeiro flatten

array_walk_recursive($array, function ($value, $key) use (&$flatted) {
    $flatted[$key] = $value;
});

var_dump($flatted);

// segundo flatten 
var_dump(array_merge(...$array));