need help for plugin

[megawelle]

hi,

let's imagine there is a template which has the variable $test.
i would like this variable to be read and altered by a plugin and make this available in the template as $test2.

if i understand it correctly, then i just need to create a plugin for that, right?

but how does the plugin get the value of the variable $test, how would i call the plugin in the template, and how does the plugin give the new variable $test2 back to the template?