As the others have stated, there are a lot of things that might affect the effort involved here. You've mentioned a desktop application -- you might start there by having your desktop app send some info to a PHP end point hosted on a server. You can read the PHP docs to get a good basic idea of GET, POST, and REQUEST.
If you want to jump straight into some framework, NogDog has suggested using Laravel, which is quite popular. In my experience, Laravel is quite heavy (just installing the framework brings in over 500,000 lines of code), and it requires you to learn 'the laravel way' of doing things.
I've done numerous projects using CodeIgniter, which is quite lightweight and has its own quirks. Lately, I've had my doubts about its long-term viability, however.