Add an array exporter and the tests.

spec/drupol/phptree/Exporter/SimpleArraySpec.php

@@ -0,0 +1,81 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace spec\drupol\phptree\Exporter;
+
+use drupol\phptree\Exporter\SimpleArray;
+use drupol\phptree\Node\ValueNode;
+use PhpSpec\ObjectBehavior;
+
+class SimpleArraySpec extends ObjectBehavior
+{
+    public function it_is_initializable()
+    {
+        $this->shouldHaveType(SimpleArray::class);
+    }
+
+    public function it_can_export_to_an_array()
+    {
+        $tree = new ValueNode('root', 2);
+
+        $this
+            ->export($tree)
+            ->shouldReturn(['value' => 'root']);
+
+        $nodes = [];
+        foreach (\range('A', 'J') as $value) {
+            $nodes[$value] = new ValueNode($value);
+        }
+
+        $tree->add(...\array_values($nodes));
+
+        $return = [
+            'value' => 'root',
+            'children' => [
+                0 => [
+                    'value' => 'A',
+                    'children' => [
+                        0 => [
+                            'value' => 'C',
+                            'children' => [
+                                0 => [
+                                    'value' => 'G',
+                                ],
+                                1 => [
+                                    'value' => 'H',
+                                ],
+                            ],
+                        ],
+                        1 => [
+                            'value' => 'D',
+                            'children' => [
+                                0 => [
+                                    'value' => 'I',
+                                ],
+                                1 => [
+                                    'value' => 'J',
+                                ],
+                            ],
+                        ],
+                    ],
+                ],
+                1 => [
+                    'value' => 'B',
+                    'children' => [
+                        0 => [
+                            'value' => 'E',
+                        ],
+                        1 => [
+                            'value' => 'F',
+                        ],
+                    ],
+                ],
+            ],
+        ];
+
+        $this
+            ->export($tree)
+            ->shouldReturn($return);
+    }
+}

src/Exporter/SimpleArray.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace drupol\phptree\Exporter;
+
+use drupol\phptree\Node\ValueNodeInterface;
+
+/**
+ * Class SimpleArray
+ */
+class SimpleArray implements ExporterInterface
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function export(ValueNodeInterface $node)
+    {
+        $children = [];
+        /** @var ValueNodeInterface $child */
+        foreach ($node->children() as $child) {
+            $children[] = $this->export($child);
+        }
+
+        return [] === $children ?
+            ['value' => $node->getValue()]:
+            ['value' => $node->getValue(), 'children' => $children];
+    }
+}