Add addNestedFields() to Form

src/Form.php

@@ -49,6 +49,39 @@ public function addField(string $name, string $content, ?string $contentType = n
         $this->fields[] = new FormField($name, BufferedContent::fromString($content, $contentType));
     }
 
+    /**
+     * Adds each member of the array as an entry for the given key name. Array keys are persevered.
+     *
+     * @param array<string|array> $fields
+     */
+    public function addNestedFields(string $name, array $fields): void
+    {
+        foreach ($this->flattenArray($fields) as $key => $value) {
+            $this->addField($name . $key, $value);
+        }
+    }
+
+    /**
+     * @return array<string, string>
+     */
+    private function flattenArray(array $fields): array
+    {
+        $result = [];
+        foreach ($fields as $outerKey => $value) {
+            $key = "[{$outerKey}]";
+            if (!\is_array($value)) {
+                $result[$key] = (string) $value;
+                continue;
+            }
+
+            foreach ($this->flattenArray($value) as $innerKey => $flattened) {
+                $result[$key . $innerKey] = $flattened;
+            }
+        }
+
+        return $result;
+    }
+
     public function addStream(string $name, HttpContent $content, ?string $filename = null): void
     {
         if ($this->used) {

test/FormTest.php

@@ -16,8 +16,36 @@ public function testUrlEncoded(): void
         $body->addField('d', 'd');
         $body->addField('encoding', '1+2');
 
+        $body->addNestedFields('list', ['one', 'two']);
+        $body->addNestedFields('map', ['one' => 'one', 'two' => 'two']);
+
+        $content = buffer($body->getContent());
+        $this->assertEquals("a=a&b=b&c=c&d=d&encoding=1%2B2&list%5B0%5D=one&list%5B1%5D=two&map%5Bone%5D=one&map%5Btwo%5D=two", $content);
+    }
+
+    public function testNestedArrays(): void
+    {
+        $body = new Form();
+
+        $body->addNestedFields('map', [
+            [
+                'one' => 'one',
+                'two' => 'two',
+            ],
+            [
+                'one' => [1],
+                'two' => [1, 2],
+                'three' => [1, 2, 3],
+            ],
+            [
+                3 => 'three',
+                10 => 'ten',
+                42 => 'forty-two',
+            ],
+        ]);
+
         $content = buffer($body->getContent());
-        $this->assertEquals("a=a&b=b&c=c&d=d&encoding=1%2B2", $content);
+        $this->assertEquals("map%5B0%5D%5Bone%5D=one&map%5B0%5D%5Btwo%5D=two&map%5B1%5D%5Bone%5D%5B0%5D=1&map%5B1%5D%5Btwo%5D%5B0%5D=1&map%5B1%5D%5Btwo%5D%5B1%5D=2&map%5B1%5D%5Bthree%5D%5B0%5D=1&map%5B1%5D%5Bthree%5D%5B1%5D=2&map%5B1%5D%5Bthree%5D%5B2%5D=3&map%5B2%5D%5B3%5D=three&map%5B2%5D%5B10%5D=ten&map%5B2%5D%5B42%5D=forty-two", $content);
     }
 
     public function testMultiPartFieldsStream(): void