How to use a function inside string?

'<a rel="nofollow" href="$1" class="bbc_link new_win" target="_blank">'

I would like use the urlencode() function:

 '<a rel="nofollow" href="urlencode($1)" class="bbc_link new_win" target="_blank">'

... but I can't use this:

 '<a rel="nofollow" href="'.urlencode($1).'" class="bbc_link new_win" target="_blank">'

... because $1 is not a variable in the string; it is instead a meta-variable in a simple free forum.

it send http://www.test.com/out.php?out=http://www.example.com