mysqli_real_connect(): Server sent charset (255) unknown to the client. Please, report to the developers
Create a file called my.cnf in your /etc/ folder.
Now add the following text to /etc/my.cnf:
[client]
default-character-set=utf8
[mysql]
default-character-set=utf8
[mysqld]
collation-server = utf8_unicode_ci
character-set-server = utf8
default_authentication_plugin = mysql_native_password
Finally, restart mysql and all should be well! If you still have an issue, try upgrading PHP to a later version.