Basic systems integrations do not require these advanced "Developer Extension" panel options. For example, if you wish to simply have text messages from users forwarded to your server as HTTP requests, you may review the section: receiving text messages.

Specify a valid URL in this field to request a callback from TextMarks servers every time a user actually subscribes (confirmed) or unsubscribes from your TextMark. This allows you to keep an up-to-date database of subscribers.

The callback URL will process tokens similar to the response URL for dynamic TextMarks (see Receiving Text Messages). However the "\a" action token will be substituted with "SUB" or "UNSUB" for subscriptions and unsubscriptions rather than the typical "REQ" for requests.

The Sub Callback URL is only called for subscribe/unsubscribe events. The callback URL for regular SMS requests is set separately in the "TextMarks Response" panel. If you wish, you may use the same URL and processing script for requests and Sub/Unsub notifications as long as you watch the \a parameter. Regardless of whether the URL is the same or not, a successful subscription will result in a SUB request. For a TextMark that is set to auto-subscribe, a single request may result in two callbacks: a "REQ" to your Dynamic URL, and a "SUB" to your Sub Calback URL.

A common template for Sub Callback URLs looks like:

Specify a secret token to request TextMarks servers to digitally sign requests (both for dynamic TextMark requests as well as Sub/Unsub callbacks). This allows your servers to verify that requests actually came from TextMarks and are not spoofed by users who somehow obtained your callback URLs.

The string resulting from URL parameter substitution of "\k\p\t" (not in URL-encoded form though) is processed to generate a keyed-Hash Message Authentication Code (HMAC) digest by using the MD5 hashing algorithm with your private token as the key. This key is made available to you in the \s signature parameter.

Most modern languages have libraries available to perform this same hash on your side. In PHP, see the hash_hmac() function. In Python, see the hmac module.