Do ‘echo’ and ‘print’ statements break my code?

I’ve been doing my first University PHP project using Xampp, and everything has been running smoothly.

I deployed my app on Google App Engine today, and it just doesn’t work. Information doesn’t pass between pages like it did when developing, and it seems like all of the ‘echo’ and ‘print_r’ functions are interrupting the app when it’s running online.

Is this likely my code? A problem with developing in Xampp? Or could it be problem with Google App Engine?