test.php

<?php

$arr1 = [[1,2,[3]],4]; 
$arr1_test = [1,2,3,4];

$arr2 = [3,[4,5,[6],7],8];
$arr2_test = [3,4,5,6,7,8];

/**
 * Given an array $arr and a new array $res, this function would flatten $arr into $res using recursion.
 */
function flatten(array $arr, &$res){
	foreach($arr as $el){
		if(is_array($el)){
			flatten($el, $res);
		}else{
			$res[] = $el;
		}
	}
}


$arr1_res = [];
flatten($arr1, $arr1_res);
assert($arr1_res === $arr1_test);
var_dump($arr1_res);

$arr2_rest = [];
flatten($arr2, $arr2_res);
assert($arr2_res === $arr2_test);
var_dump($arr2_test);

?>