solution.php

solution.php

<?php declare(strict_types=1);

$sections = json_decode(
	<<<'JSON'
		[
		  {
		    "title": "Getting started",
		    "reset_lesson_position": false,
		    "lessons": [
		      {"name": "Welcome"},
		      {"name": "Installation"}
		    ]
		  },
		
		  {
		    "title": "Basic operator",
		    "reset_lesson_position": false,
		    "lessons": [
		      {"name": "Addition / Subtraction"},
		      {"name": "Multiplication / Division"}
		    ]
		  },
		
		  {
		    "title": "Advanced topics",
		    "reset_lesson_position": true,
		    "lessons": [
		      {"name": "Mutability"},
		      {"name": "Immutability"}
		    ]
		  }
		]
	JSON,
	true,
);

$lessonPosition = 0;
$result = array_map(static function (array $section, int $sectionPosition) use (&$lessonPosition): array {
	$lessonPosition = $section['reset_lesson_position'] ? 0 : $lessonPosition;
	return ['position' => $sectionPosition + 1] + ['lessons' => array_map(static function (array $lesson) use ($section, &$lessonPosition): array {
		return ['position' => ++$lessonPosition] + $lesson;
	}, $section['lessons'])] + $section;
}, $sections, array_keys($sections));

var_dump($result);