array-flat.php

<?php
/*
	Result:

	Array
	(
		[foo.bar] =>
		[foo.down.a] => AA
		[foo.down.b] => BB
		[value] => test
	)
*/

$array = [
	'foo' => [
		'bar' => '',
		'down' => [
			'a' => 'AA',
			'b' => 'BB',
		],
	],
	'value' => 'test',
];

function flat(array $array, string $prefix = ''): array
{
	$result = [];
	foreach ($array as $key => $value) {
		if (is_array($value)) {
			$result = array_merge(
				$result,
				flat($value, $prefix . $key . '.')
			);
		} else {
			$result[$prefix . $key] = $value;
		}
	}
	return $result;
}

var_dump(flat($array));