Description:
------------
It would be immensely useful to have a source filtering capability similar to Perl:

http://perldoc.perl.org/perlfilter.html

Essentially, a way to allow a special block of code to execute and "filter" through the source code of the script before it is fed through the interpreter.

This would allow some very useful features, such as embedding debugging code within comments, and then automatically executing that code if a debugging filter is included into the script.  Thus, if the filter is left out, the debugging code remains harmless comments:

<?php
include('debug.php');
$result = some_convoluted_function();
### Expected: 106852.244
### Got: $result
?>

This particular kind of example is based off the Perl Smart::Comments module:

http://search.cpan.org/search?query=Smart%3A%3AComments&mode=module