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I don't claim to be an expert. I was having the same error
than I saw this comment in the install.txt right up top.

QUICK START INSTRUCTIONS

By default PhpWiki is configured to use a flatfile
database. If there
is no dbm on your system, you will see an error like this
the first
time you try to use PhpWiki:

"Fatal error: Call to undefined function: dba_open() in 
 phpwiki/lib/DbaDatabase.php on line 32"

To correct this you will have to install a dbm package or
preferably
an sql-compatible database such as MySQL or PostgreSQL and
make the
necessary configuration changes to index.php.