Flatten array

tests/FlattenArrayTest.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+use function App\flattenArray;
+
+it('flattens an array', function (
+    array $input,
+    array $expected
+): void {
+    $result = flattenArray($input);
+
+    assertSame($expected, $result);
+})->with([
+    [
+        'input' => [1, 2, 3],
+        'expected' => [1, 2, 3],
+    ],
+    [
+        'input' => [1, [2, 3, null, 4], [null], 5],
+        'expected' => [1, 2, 3, 4, 5],
+    ],
+    [
+        'input' => [1, [2, [[3]], [4, [[5]]], 6, 7], 8],
+        'expected' => [1, 2, 3, 4, 5, 6, 7, 8],
+    ],
+    [
+        'input' => [null, [[[null]]], null, null, [[null, null], null], null],
+        'expected' => [],
+    ]
+]);