As a tool for very specific projects - projects which are generally web projects that will never go beyond a front-end layer and require much complexity in the backend pieces. However you have to know in advance that you are writing a project that is specifically like that - or you can write zend extensions in C or use something like Thrift to talk to non-PHP back-end pieces.

(My own preference is not to use PHP whenever possible for personal projects - but I run my own servers and am fairly familiar with mod_perl, FastCGI and Apache internals).