I think there's problem but it's elsewhere. Sure it's a single file, yes. However in order to run it you need a php interpreter, which I for example, do not have installed and an internet connection. But that would go for just about every scripting language in one form or another. Sure, just about every linux distribution has python for instance and same goes for macos afaik. This isn't the case for windows however(again afaik). Package it into a single binary file and I'd be all up for it: I do love AppImages.
> that would go for just about every scripting language in one form or another
You could use bash and be supported on almost every unix-a-like OS¹, or plain-'ol sh for even wider compatibility. You'd need to configure CGI/FCGI as well as dropping the script(s) in, but you have to do similar work for PHP/python/other if you don't already have them installed and configured for the site/directory in question.
[1] and others: it'll work with IIS if the server has a bash/sh interpreter² available
[2] plus any externals that the script relies upon, of course
