I have a bunch of separated PHP classes. They have nothing special, no constants, just normal variables. The only connection between them are the call of methods.
Is there a way to model this connection in the UML class diagram? I guessed that it would be the "association" with a -> arrow
Correct. If A2 has a reference to B2 (i.e., using a "normal variable"), you write the name of that variable at the end of the association, nearest to the arrowhead pointing to B2. If that variable references a collection of B2s, you ignore the intervening collection type and make the multiplicity [0..*].