MediaWiki 1.5.6 and 1.4.14 are security and bugfix maintenance releases.
A bug in edit comment formatting could send PHP into an infinite loop
if certain malformed links were included. In most installations, this
would cause the script to fail after PHP's 30-second failsafe timeout.
in 1.5.6, some improvements have been made to the installer which should make
installation possible on a system with a broken MySQL "root" account. Some
miscellaneous fixes are also present; see the complete change list at the end of
the release notes, linked below.
1.3.x releases are also vulnerable to the loop bug, but we are no longer
maintaining that branch, and it doesn't work with current versions of PHP. Older
installations probably should consider upgrading for the numerous bug fixes etc.