<?php

$results = [
	['id' => 1, 'breakdown' => [1, 2]],
	['id' => 2, 'breakdown' => [5, 6]],
	['id' => 3, 'breakdown' => [7]],
];

$extraAccessorials = [9];

foreach ($results as $key => $value) {
    $results[$key]['breakdown'] = \array_merge($results[$key]['breakdown'], $extraAccessorials);
}

var_dump($results);exit;