I found a solution for this problem after a long analysing procedure.
After properly testing my php installation with the command line features I found out that the php is working well and could work with the mysql database. Btw. you can run code-files with php code with the command php -f filename.php
So I realized, it must something be wrong with the Apache.
I made a file with just the phpinfo() function inside.
Here I saw, that in the line
Loaded Configuration File
my config file was not loaded, instead there was mentioned (none).
Finally I found within the Apache configuration the entry
<IfModule php5_module>
PHPINIDir "C:/xampp/php"
</IfModule>
But I've installed the PHP 7 and so the Apache could not load the php.ini file because there was no entry for that.
I added
<IfModule php7_module>
PHPINIDir "C:/xampp/php"
</IfModule>
and after restart Apache all works well.
These code blocks above I found in my httpd-xampp.conf file. May it is somewhere else at your configuration.
In the same file I had changed before the settings for the php 7 as replacement for the php 5 version.
#
# PHP-Module setup
#
#LoadFile "C:/xampp/php/php5ts.dll"
#LoadModule php5_module "C:/xampp/php/php5apache2_4.dll"
LoadFile "C:/xampp/php/php7ts.dll"
LoadModule php7_module "C:/xampp/php/php7apache2_4.dll"
As you can see I have the xampp package installed but this problem was just on the Apache side.
new mysqli()
– undefined