Неправильная полезная нагрузка JSON Slack Webhooks Переполнение стека

Slack Incoming Webhooks возвращает полезную нагрузку не в формате JSON, и документация не имеет особого смысла.

public function index() {

$message = [
"pretext"    => "There's been a sale on Example Cart",
"title"      => "Sales Order #123456 on Example Cart",
"title_link" => "http://www.example.com/admin/index.php?route=sale/order/info&order_id=123456",
"text"       => "There's been a new sale on the Example website. Click the order above to check it out.",
];

$send = self::slack($message);

dd($send); // die and dump curl result

}

public static function slack($message, $room = "test-messages", $color = "#ED7100", $icon = ":mailbox:") {

foreach ($message as $key => $value):
$message[$key] = str_replace(["<", ">", "&"], ["&lt;", "&gt;", "&amp;"], $value);
endforeach;

$fallback = $message["pretext"] . " - " . $message["title"] . " - " . $message["title_link"];

$attachments = [
"fallback"   => $fallback,
"pretext"    => $message["pretext"],
"title"      => $message["title"],
"title_link" => $message["title_link"],
"text"       => $message["text"],
"color"      => $color,
];

$payload = [
"channel"     => "#{$room}",
"channel"     => "@bossman",
"icon_emoji"  => $icon,
"username"    => "webhook-tests",
"attachments" => [$attachments]
];

$data = "payload=" . json_encode($payload);

$ch = curl_init("https://hooks.slack.com/services/endpoint");
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$result = curl_exec($ch);
curl_close($ch);

return $result;
}

Когда я вставляю JSON в один из онлайн-линтеров, он возвращается как действительный JSON.

Что ищет Slack, которого нет в документации?