Core question at hand: which SE site(s) are phpMyAdmin usage questions considered to be most topical?
Effective September 1st, 2013, support for questions directly related to phpMyAdmin has moved to stackoverflow. All prior support mechanisms have been phased out.
Questions about server issues (web server configuration, "404 - Not Found" error, etc), should be asked on serverfault.com.
The way of reporting (security) bugs and adding feature requests will be unaffected.
According to Is it okay to use Stack Overflow as the support forum for a product or project?, I can't tell if this is abuse or if this is acceptable use, especially since this is a community-driven project.
Questions related to setting up or configuring phpMyAdmin in a business/professional environment are considered on-topic at SF. What sites should the other classes of questions be asked on?
SO is supposed to be for dev/programming only, and phpMyAdmin is often used outside of this context, making it an inappropriate forum in which to ask usage questions in many cases.
For phpMyAdmin usage, https://dba.stackexchange.com/ seems like the logical choice, though it's often used by non-DBA role persons, such as SysAdmin generalists and webmasters, where DBA is a task rather than a role, which drags SF and Webmasters into consideration.
Some data on search results for "phpmyadmin" by site:
- SO: 37,636 results
- SF: 1,647 results
- DBA: 629 results
- Webmasters: 237 results
- SU: 551 results (just for completeness, though some questions may "belong" here)
Points in favor of posting to DBA:
Database Administration including configuration and backup / restore
Tools are officially on-topic here but of course it's a borderline area where there is overlap with SF, SO and even SU
Points in favor of posting to Webmasters:
Questions here are commonly about[ ... ]web hosting
Points against posting to Webmasters:
There aren't any results for phpMyAdmin in the meta.
There are some types of questions about running websites that don't belong on this site:
System and network administration -- Server Fault participants are much more likely to know the technical details on topics such as DNS and Apache configuration than people who answer questions here.
SQL administration isn't mentioned here, but website backing databases aren't the only thing that phpMyAdmin users use phpMyAdmin for, and so would be off topic in at least those cases.