The actual solution is more simplistic. Those methods simply need to be made static. The class has no members, so there's no need to instantiate it. That's why every time someone posts a "solution", we tell you not to use it. phpBB 3.0.x officially still supports PHP 4, so we cannot make those functions static, but we were able to in 3.1.x.
The correct fix:
Find: (lines 80, 122, 154, 186, 212, 245, 947)
However, if you are receiving these warnings, it is a sign that your installation is either out-of-date or has not been properly updated.