Лексей Фельде

class User {
...
    /**
     * @Serializer\VirtualProperty()
     * @Serializer\SerializedName("tags")
     */
    public function getTags()
    {
        return explode(',', $this->tags->getStr());
    }
}

function toHtml(array $content) {
    $html = '';

    foreach ($content as $row) {
       if (is_string($row)) {
            $html .= $row;

            continue;
        }

        $attrs = '';
        if (isset($row['attrs'])) {
            foreach ($row['attrs'] as $name => $value) {
                if ($row['tag'] === 'img') {
                    $value = '//telegra.ph' . $value;
                }
                $attrs .= sprintf(' %s="%s"', $name, $value);
            }
        }

        $children = '';
        if (isset($row['children'])) {
            $children .= toHtml($row['children']);
        }

        $html .= sprintf('<%s%s>%s</%s>', $row['tag'], $attrs, $children, $row['tag']);
    }

    return $html;
}

$content = json_decode(
    file_get_contents('https://api.telegra.ph/getPage/Vozmozhnosti-Telegram-03-30?return_content=true'), 
    true
);

echo toHtml($content['result']['content']);