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The PHP "mysql" extension is not developed anymore, and could be deprecated any time now. So it would be positive to start using the "mysqli" extension (mysql "improved").
I've run XOOPS 2.6.0 through a converter from Oracle:
https://wikis.oracle.com/display/mysql/Converting+to+MySQLi
and have created a test branch for it:
https://sourceforge.net/p/xoops/svn/10359/tree/XoopsCore/branches/2.6.x/2.6.0_mysqli/
The converted XOOPS 2.6.0 seems to be working fine on PHP 5.4.8 with "mysql" extension turned off, but I need some DB gurus to go through all the warnings and see what else should be changed/adjusted manually (see the readme_myqsli_conversion_warnings.txt file).
Once the conversion is properly done and finalized, we could think about converting all DB calls to OOP style using mysqli, and then to PDO
But the first thing is to make sure that XOOPS 2.6.0 works properly with "mysqli" extension.