HTTPRequest class doesn't exist in PHP 7+
The HTTP extension is a PECL package that needs to be installed separately.
This is 'difficult to impossible' to accomplish on shared hosting platforms
$request = new HttpRequest();
However, the https://v3.developer.constantcontact.com/ guide consistently uses HttpRequest() as the example.
Test, confirm and show real world working examples using cURL instead.
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
The request for V3 application code samples is currently under review. However, we're currently still adding endpoints and capabilities to the V3 API, and do not plan to publish official SDKs or sample apps until that process is complete. Your feedback and experience with this request is essential to improving our product, so thank you for reaching out to us regarding this matter.
In the meantime, here are the example calls that are currently available for V3: