there is much more than just providing OS source code. I personally think that single files are going to be overkill and the "review" will not happen because it will be too much.
On the other side, the source code is on GitHub. You (and many other developer) are welcome to review. GitHub would not allow malicious code to be hosted on their platform. And in the end, it is the same process as you would review single files..
Further, there is also the aspect of dependencies: the power of OS software provides the ability to use other OS libraries. This means that I do not need to reinvent the wheel and use existing solutions. Providing single files would require that you make a bunch of 'include' statements before you can run my code. This is pain..
And finally, I - as the creator - guarantee that PHPAlgorithms does not do malicious things :)
PR's welcome :)