How to integrate backend language with frontend. [Beginner]

I have started developing a webapp that let you upload a .csv file, elaborate it and insert the data into a database. The csv data is elaborated to a json to extract easier the data, but everything is done with php.
I studied java and I can use it as a backend language, but I have literally no idea how to integrate the part that elaborates the data (that would be backend, so done in java) with the part that takes the file (that is php).

submitted by /u/imprudenza
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