I have just installed XAMPP 1.7.3 on a mac running High Sierra 10.13.2. Straight away, when I visit http://localhost/phpmyadmin, I get this error:
#1273 - Unknown collation: 'utf8mb4_unicode_ci'
If I use 127.0.0.1 instead of localhost, then it works fine. I have not created any databases yet, just the default XAMPP install.