In your case you shouldn't. :) PHP is string manipulation language, it's really hard to do anything else with it. (binary manipulation, network programs, binary streams, long running apps...)
Also in practice PHP has been weak at certain types of string manipulation, because the template language doesn't lend itself to capturing the output in order to filter it. Intermediate PHP developers know how to save buffered data, but it isn't as straightforward as manipulating output data with rack or wsgi middleware.
No no no no no no no. Nonono. No.
It's a web framework and template language built around a handful of C wrappers and awkward database interactions. It can't even represent unsigned 64 bit integers. Any complex math requires you to convert the numbers into strings.
PHP is an advanced template language for HTML.
PHP is a template language for HTML that's prone to CSRF.