To say that there's a "killer app" for this or that programming language would mean that the language has a certain "specialization" that makes it uniquely suited to a particular use.
Well, I do think it worked for PHP -- somehow, PHP for a lot of people really caught on as the thing to write dynamic webpages in. And while some people also try to do system admin and cron stuff in it (you will recognize the beginner by seeing commands like 'lynx -dump http://mydomain/script.php' in his cron table), (fortunately) it is mostly limited to the web right now.
I never quite understood why -- I guess if PHP would not have the '<?' and '?>' tags, but would per default work like perl (ie., use an extra module/different workflow to embed code into HTML), it would not really have caught on.