PHP Syntax Question

I posted on the author’s site, and I understand that he cannot respond to questions about PHP shortcode. But perhaps someone in the wider WordPress community would help me with this one:

The variable in $phil_img is the url to a GIF image. It works fine in url building, and populating <img ...> tags, etc… But I am trying to code in a conditional test:

IF there is no GIF image, THEN I want to use a default “generic.gif”.

But I’m having trouble getting the syntax for conditionals that is parsed properly by the plugin.

Should the following work? (It does not.)

if(!file_exists($phil_img)) { return 'That is not a file.'; }

Thanks for any pointers.
~Brendan

http://wordpress.org/extend/plugins/shortcode-exec-php/