So I'm trying to develop some advanced bbcode functions (well alright, not that advanced

) which I would like to use non hard coded values in, e.g. vboptions[bburl]. I WON'T be using complex php arguments that can be security risks.
Obviously I could add a replacement variable to go with the release, replace *Borgs_hack_your_forum_path_here* with your forum path in the replacement variables, but that makes thinks slow to install. Plugins are faster.
How could I run a plugin to replace some text? I've been investigating str_replace without much joy, I guess it could be a global replace as if I keep the value unique enough it'll never clash with anything else.
Do any php coders know how to make a replacement variable into a plugin?