What is the best way to: How to fix a plastic key holder which is broken or split?

Choices:
-- Heat the plastic with a small flame and once hot, press the two parts together and hold them in place, one you remove them they should hold together again
-- Heat the plastic with a large flame and once melting, press the two parts together and hold them in place, one you remove them they should hold together again
Answer:
Heat the plastic with a small flame and once hot, press the two parts together and hold them in place, one you remove them they should hold together again