The library can be found on Composer as lowentry/ue4-php.
The library is being versioned as X.Y.Z, where:
Z is the incremental or fix number, when a bug has been fixed, or a performance issue has been addressed, this number will increase. This should never change anything to the library's API, meaning this should never break code that relies on this library.
Y is the minor or feature number, when a new feature has been added to the library, this number will increase. This could add to the library's API, but it should never remove or break anything from the (previous) library's API, meaning that this too should never break code that relies on this library. These updates can however deprecate code that will be removed in a future major update, which can cause compiler warnings.
X is the major or incompatibility number, when the library's API has changed in a way that it could break code that relies on this library, this number will increase. The breakage that happens usually isn't that big, but it does mean you can't just upgrade this library and expect your code to automatically still work, you will probably need to fix a couple compiler warnings or errors here and there.