How I can call REST API?

In PHP classic how I can call REST API ?

You can do that by making an AJAX request using jQuery.

You can find examples here, used on PHPRad Classic 2.7.3: Adding cusotm button php (phprad.com) and Trigger Update single field in Controller.php (phprad.com)

You can read more here jQuery AJAX Methods (w3schools.com) and here jQuery.ajax() | jQuery API Documentation