Usually mga generic na mga apps like HR, Accounting, CRM, Hotel, meron mga open source. Baka may mahanap ka, google mo lang like "library management system php github".
If galing ka na sa conventional web programming like Php at familiar ka na sa mga frameworks, i recommend to start using Laravel Framework. Instant code scaffolding, highly maintenanable, good separation of concerns (MVC), eloquent, opensource at madali sya matutunan kahit manood ka lang ng tutorials sa YøùTùbé then sabayan mo ng code.