When thinking about a REST API and PHP, it's important to first understand that a PHP application can answer a REST API GET request, providing data to the requestor.

Or a PHP application can submit a GET request, requesting data from a REST API.

Answer a REST API GET request

Creating a REST API with PHP is actually incredibly easy. For example, www.freekb.net uses PHP to produce web pages. Let's say foo.php contains the following.

<?php
  echo "Hello World";
?>

Navigating to http://www.freekb.net/foo.php displays Hello World.

Likewise, using a REST API, such as the curl command, submitting a GET request to www.freekb.net/foo.php also returns Hello World.

~]$ curl --request GET --url http://www.freekb.net/foo.php
Hello World

Almost always, a REST API returns JSON formatted data, so it probably makes sense for foo.php to return JSON formatted data.

<?php
  $json = '{"foo":"Hello","bar":"World"}';
  echo $json;
?>

Which should return the JSON data.

~]$ curl --request GET --url http://www.freekb.net/foo.php
{ "foo": "Hello", "bar": "World" }