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bug 26690) Value of the 'Name of wiki' field is not retain after two subsequent click on 'Continue' button. This has been a complaint since the beginning, that the installer "forgets" $wgSitename.
The original intention, as I saw it, was to encourage people from choosing 'MediaWiki' as the name of their wiki. Just hardcode it, I don't think we're changing the name anytime soon.