Do you know any financial support for international PhD students to cover Article processing charge (publication fee)? Specifically, for PhD students from developing countries studying in Europe.
I am not aware of funding agencies in your geography but at a national level, you can find some such agencies. In India, ICSSR sponsors research scholars to write research papers and even fund for research. All they expect the author is to mention the name in the Acknowledgement section.
Many journals and publication networks also have a differential fee structure and even waiver for certain countries.