Need to call php function from bbcode

[arob42]

I want to take the contents of a custom bbcode and send those contents as a parameter to a php function and use the output of the php function as the replacement for the custom tag contents.

As an example, consider the following line of text:
Blah blah [newtag]blah[/newtag] blah.

using a replacement similar to this: php_function( {param} ) I want the text to be changed to the following:

Blah blah <output of php_function here> blah.

The output of the php function will be a image reference. i.e. <img src="...">.

Can I do this using existing tools or must I hack functions_bbcodeparse.php

Thanks!