[Recommendation]making a realtime application with php. ajax vs websocket what should i use

GREET!! I am in a assignment to make real time chat like system with php. I am in a round middle of that by using ajax. I send request to server in evvery short delay and server responds. Although my friend later suggested me to use websocket instead because this is not good when there is high traffic..he suggested. What I am supposed to do later on...