MAMP phpMyAdmin is not active?

Okay, so I finally solved the problem. First as aynber mention above in the comment section, I needed to install another Php version, so with homebrew, I installed the PHP 7.0;

brew install php70

then to start;

brew services start homebrew/php/php70

and then;

export PATH="$(brew --prefix homebrew/php/php70)/bin:$PATH"

finally, I changed the folder name of the PHP 7.1.1 in MAMP folder to

/Applications/MAMP/bin/php/php7.1.1_notActive

after that restarted the MAMP and now I have access to phpMyAdmin.

Selecting 7.0.15 in the preferences worked for me, but also...

Just going to http://localhost/phpmyadmin/ opens phpMyAdmin on my MAMP install running 7.1.1

You can choose another PHP version from Preferences -> PHP.

There is PHP 7.0.15 shipped with MAMP. Enable it and restart MAMP.

MAMP shows only two versions of PHP, so if you don't see it on the list then go to the folder

Applications/MAMP/bin/php

and rename folders with PHP versions you are not using.

Open terminal. Then open with sudo this file /Applications/MAMP/bin/mamp/index.php in your favorite editor, for example:

sudo nano /Applications/MAMP/bin/mamp/index.php

Then find that text block (it occurs twice in the index.php, for the menu item and for the link):

<?php if(version_compare(PHP_VERSION, '5.5.0', '>=') and version_compare(PHP_VERSION, '7.1', '<') ): ?>

and remove last php version check like this:

<?php if(version_compare(PHP_VERSION, '5.5.0', '>=')): ?>

Save file and restart MAMP, not only Apache, but also MAMP control center.

Profit!

P.S.: for nano, you can find desired string with holding ctrl+w, paste if(version_compare(PHP_VERSION, '5.5.0' and pressing enter.