I have a question about how actions and filters interact with functions:
I am currently using an event ticketing plugin for wordpress, there are built in validation rules that is already in the plugin (i.e. fields are all completed, number fields contain integers etc.). I want to write a function that 'extends' the existing plugin function. When I write my functions, does the existing plugin function for that hook still apply? Or would I have to copy over parts of that function I want to retain into my new one? Or does wordpress run both functions in some kind of sequence?
If I wanted to write a validation that is the opposite of what is currently in the plugin function (i.e. number fields should contain an alphabetical character) how does a custom function overwrite a part of an existing function? Would I have to remove those lines in the plugin file itself?
Thanks in advance.