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function getCombinations($arrays) {
	$result = [[]];
	foreach ($arrays as $property => $property_values) {
		$tmp = [];
		foreach ($result as $result_item) {
			foreach ($property_values as $property_value) {
				$tmp[] = array_merge($result_item, [$property => $property_value]);
			}
		}
		$result = $tmp;
	}
	return $result;
}

$combinations = getCombinations([
   1 => ['a1', 'a2', 'a3'],
   2 => ['b1', 'b2', 'b3'],
]);