Joomla 2.5 and PHP 5.6

I have older Joomla 2.5 website I want to load (does not support PHP 7) I have to use PHP 5.6. Created new db user & db, and imported db via phpmyadmin from old backup, as well as moved all of the files via FTP. For this website, I changed from default PHP to Fast-CGI
PHP 5.6
DB Username, DB password, DB host info are correct.

I moved many sites and loaded them to my server without issues. I'm not sure if this issue is related to my PHP 5.6 additional install. Any ideas where to look and how to fix. When I go to the website I get:

Error
Database Error: Unable to connect to the database:Could not connect to MySQLDatabase Error: Unable to connect to the database:Could not connect to MySQL

Error Logs:
[Thu Feb 20 10:02:52.670261 2020] [fcgid:warn] [pid 23537] [client 12.12.12.12:42994] mod_fcgid: stderr: PHP Notice: Constant JDEBUG already defined in /var/www/clients/client37/web63/web/includes/framework.php on line 143

When I change website PHP from
Fast-CGI
PHP 5.6
to

PHP-FPM
PHP 5.6

I get

Service Unavailable
The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

Additionally, a 503 Service Unavailable error was encountered while trying to use an ErrorDocument to handle the request.

Apache/2.4.25 (Debian) Server at mydomain.com Port 443

 

Have you installed php-mysql for PHP version 5.6? Did you check twice the paths for 5.6 are correct?

 

Thank you @Taleman.

I reinstalled PHP-5.6 from this link
modified paths inside of ISPConfig, restarted service, and it works now.
I think i had php-5.6 installed twice compiled manually and from Sury.org.

now error logs shows
[Thu Feb 20 13:11:20.239328 2020] [proxy_fcgi:error] [pid 16527] [client 12.12.12.12:46318] mod_fcgid: stderr: PHP Fatal error: require_once(): Failed opening required '/var/www/clients/client37/web63/web/libraries/import.legacy.php' (include_path='.:/usr/share/php') in /var/www/clients/client37/web63/web/includes/framework.php on line 33

 

Does this file exist and has the right permissions?

 

After I installed following packages

apt-get install php5.6-mysql
apt-get install php5.6-mbstring

website started working...