Empty delimiter error - ??

I'm totally unfamiliar with php, and making some slight modificiations to Chris Heilmann's nice v 1.1 Newsleterscript. What exactly is meant by an empty delimiter? This code works just fine, but I get the following error: Warning: Empty delimiter in /home/sites/site56/web/trailcrew.php on line 63 # Put the entries into the array lines $lines = explode("%",$content); for ($key=1;$key<sizeof($lines);$key++){ # when the email is not in the list, add the old entries if (!stristr($lines[$key], $email)) { <---- offending line - line 63 $out .= "%".$lines[$key]; } # when it's already in the list, set found else { $found=1; } } Thanks Rich Pinder