I had the need to integrate a PHP web app with Salesforce, and given the humongous popularity of the platform, it was surprising that there was no library which provides a seamless way into its REST API.
Google results returned these two packages, but neither was being actively developed, nor were they up to date with the recent state of Salesforce's API, and both were using a non-recommended authentication flow.
Via Composer
composer require xhezairi/sforce-rest-api-php