If you want to use ExpressCheckout via the REST API from Paypal, you can follow these steps to get a working API within your webapp.
1. Install Composer, Curl and PHP on a Webserver
2. a) Create a composer.json file with following data from the top of this page: https://developer.paypal.com/webapps/developer/docs/api/
2.b) Install the libraries with composer install
2.c) Include the autoload.php from the Composer installation with a require_once.
3.a) Setup the REST-API-SDK-PHP library files with your credentials from the Paypal developer site: sdk_config.ini and in the bootstrap.php that could be found in the folder vendor/paypal/rest-api-sdk-php
3.b) Set up a define command with a pointer to the sdk_config.ini file. define(‘PP_CONFIG_PATH’, …path to the sdk_config.ini file…);
4. Build a form with a PayPal-button and send the form parameters via $_POST to this file: https://github.com/paypal/rest-api-sdk-php/blob/master/sample/payments/CreatePaymentUsingPayPal.php
5. After successful payment from the Paypal website you have to redirect the user to this file: https://github.com/paypal/rest-api-sdk-php/blob/master/sample/payments/ExecutePayment.php
You are done!