PHP is a niche language, the niche is probably best described as "for want of something else that seems so easy to use and has such a low barrier to entry we are forced use it for web development".
The interesting thing here is that apparently the difference between PHP and the 'rest' of the languages out there is not large enough to supplant PHP in the WWW domain.
It's close enough to C that a whole generation of C programmers is intuitively at home in PHP, until they start using its advanced features.
But as long as you have to do simple stuff and get paid for that it almost always works. The few times that it doesn't you wished you never knew about the language though.
Just imagine, a language where the default 'scope' is global but globals are not available to functions and silently get replaced with empty variables. Who came up with that gem ?