Background information: I am implementing one system on two programming languages - Java and PHP. Some of the functions will be implemented on Java, and the rest of the functionalities will be implemented using PHP.
I am actually new to UML diagrams and I am not sure if I can use the same diagrams (sequence, use case, class) to explain about my system.
Apologies if I did not phrase my question in a clearer manner. This is the best I can think of.
UML diagrams are intended to be abstracted from things like the language used to implement the system; this level of abstraction is one of the reasons to use UML.
The use case diagram (reference from comments) expresses what can be done from the actor's point of view, but that's not related to the language chosen for the implementation. The language (and ways that language may help accomplish the use cases) is an implementation detail and should remain decoupled from the system features.