Why does MAMP not change the php version
I was able to change the PHP Settings after changing the available PHP versions that MAMP offers. I did this by going to "/Applications/MAMP/bin/php" and renaming one of the folders, for example adding "_X" at the end. By renaming the folder of one of the 2 available PHP versions you remove it from the selection. Afterwards I was able to change and save the standard PHP version.
A bug was recently submitted on Mamp's Bugbase but it is unresolved at this moment. Meanwhile, the solution described by koedev, as well as the solutions found at the workaround cited by Arda Kazanci could be used. Globally, two different solutions are available in the workaround cited by Kazanci: The koedev solution, that is, adding a _X in folder's name, and a second one that seems to be more professional, that is, adding an "extra" folder to httpd.conf and "selecting" inside which php versions should be used by mamp. Because some users didn't succeed in making work the extra configuration of httpd, it may be faster and simplest to follow the koedev solution.