PHP 5.6 multiversion on Debian 9 cannot connect to the database

Hi,
in a production web server with native Debian 9 Stretch and ISPConfig 3.1.13 (Apache) I installed the additional PHP versions following the how-to https://www.howtoforge.com/tutorial/how-to-install-php-5.6-and-7.1-on-debian-9-stretch/
For PHP versions 7 (so the default 7.0, the 7.1 and 7.2) no problem.
Also version 5.6 seems to work well (both Fast-CGI and PHP-FPM) but when a website needs a database (MariaDB) connection, e.g. for WordPress but not only, this error is returned: "Error establishing a database connection".
On the page phpinfo, PHP version is correct (5.6.33) and the mysqli function is active (mysqlnd 5.0.11-dev - 20120503).
What can I do to solve the problem?
Thanks a lot

 

The file log/error.log are empty.
Activating WordPress debug mode, in the wp-content/debug.log there is this error:
[19-Oct-2018 07:39:51 UTC] PHP Warning: mysqli_real_connect(): (HY000/2002): No such file or directory in /var/www/clients/client33/web63/web/wp-includes/wp-db.php on line 1531

The line 1531 on wp-db.php is this:
mysqli_real_connect( $this->dbh, $host, $this->dbuser, $this->dbpassword, null, $post, $socket, $client_flags );

If I change Fast-CGI (or PHP-FPM) on PHP 7.x the same website works fine.

May I try to install PHP 5 on the system (apt-get install php5 php5-mysql)?
Because in the open_basedir (phpinfo) there is also /usr/share/php5 folder (that doesn't exist on the file system) and there is not /opt/php-5.6 folder, is it normal?

 

Following your link I completed these two variables in the /opt/php-5.6/lib/php.ini:

• pdo_mysql.default_socket = /var/run/mysqld/mysqld.sock
• mysqli.default_socket = /var/run/mysqld/mysqld.sock

For new versions of Wordpress now the problem is fixed, but not for the version 3.2.1.
I have to transfer a site in Wordpress 3.2.1 that works perfectly on another web server, almost identical to the new one: the only difference is that the old server started with Debian 8, and then PHP 5, only to be upgraded to Debian 9 with PHP 7.
On that server, same site in Wordpress 3.2.1 with PHP version 5.6.33 it runs smoothly.

Any other idea?

 

Yes, it's the first solution that I tryed but works only for the new versions of Wordpress, not for the 3.2.1.

I solved following the how-to https://tecadmin.net/install-php-debian-9-stretch/
I installed only php5.6 (put the 7.1 and 7.2 in the /opt folder is better and works fine).

I added php5.6-fpm and php5.6-cgi modules (they are not present in that how-to) and I compiled the fields on ISPConfig -> System -> Additional PHP Versions without problems.

The php5.6 package by sury.org behaves exactly like php7.0, it doesn't open php5 folder but put the files in /etc/php/5.6 and in /usr/share/php5.6-module.

This way works and it's even easier as an installation, do you think there may be problems in production?

Thanks a lot