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A convenient, fast and secure PHP forum engine for docking with an existing PHP 5.4 project. Which to choose?

Welcome all!
If anyone has recently installed (or just knows) a convenient, fast and safe forum engine in PHP for docking with an existing project, please tell me.
What is the best forum engine to choose now to support users (support forum) of a long-standing existing project?
Now the project is on PHP 5.4 and there is no way to upgrade for a number of reasons.
You need a new one to "dock" to the project instead of phpBB and make integration with "transparent" authorization (so as not to understand the code for a week).
The reason for replacing phpBB : after disabling eval() in php.ini (after a series of attacks), everything failed in the phpBB forum engine that stood before, because. his code is simply "stitched" through them!
Thank you all very much for your help!