The coolest trick I have seen (besides the obvious ones) is simply by adding an "@" in front of the function that is causing the issue.

Please note: this is just to HIDE the warning.
It is better to fix the issue ... but sometimes the issue cannot be fixed right away.

How to use this:

Say you have the functions (I'm showing 2 examples)

dosomething();

$xyz = calculatesomthing();

And these function throws a warning.
To suppress the warning:

@dosomething();

$xyz = @calculatesomthing();