I have just installed PHP5 on my local development server, and would to share these impressions with everyone.
First of all - latest version of miniBB works with PHP5 without any problems! I have turned on default mySQL module in php.ini (uncommented extension=php_mysql.dll - by default, it's turned off) and copied libmysql.dll to Windows/System32 folder. In combination with other settings, this all allowed my old good Apache 1.3.33 successfully run PHP5 with no troubles.
I must say that some addons and probably language packs will cause execution of miniBB because of incorrect syntax of using php tags like <?php ... ?> From old times, we have marks like <? ... ?> left in many scripts. By default, PHP5 configuration allows only <?php ... ?> and <script> tags, so every script containing <?...?> tags will not execute, and can cause the general execution, if it's being included. Overall, main miniBB core and all common (latest) addons contains <?php...?> tags, and everything is working just fine.
Also, global arrays like HTTP_POST_VARS (also as for GET and COOKIE) are not available anymore. So, miniBB 1.x will not work, but 2.x will.
I have not tested mysqli yet, but this is what you can get from the solution above...
Additionally, miniBB 2 RC6 is on the way, and we hope this will be really latest release of the beta series. We are working now on XHTML compliant templates, and this is the main goal we want to dedicate all further development, incl. renovation of addons, CSS skins and language packs.