What are the two types of codes mentioned in the thread and how can they be converted in PHP?

The two types of codes mentioned in the thread are HTML entities and URL encoding. HTML entities are used to represent special characters in HTML, while URL encoding is used to encode special characters in a URL. To convert HTML entities to their corresponding characters in PHP, you can use the `html_entity_decode()` function. To convert URL encoding to plain text in PHP, you can use the `urldecode()` function.

// Convert HTML entities to their corresponding characters
$html_entity = "<p>Hello, world!</p>";
$decoded_html = html_entity_decode($html_entity);
echo $decoded_html;

// Convert URL encoding to plain text
$url_encoded = "Hello%2C%20world%21";
$decoded_url = urldecode($url_encoded);
echo $decoded_url;