Add .phps extension for PHP

.phps stands for "PHP Source"

Useful to show examples of PHP code. A properly-configured server will
output a .phps file as is, with color-formated source code instead of
the HTML that would normally be generated.

Usage in GitHub:
https://github.com/search?utf8=%E2%9C%93&q=extension%3Aphps+NOT+nothack&
type=Code

Example taken from:
https://github.com/PHPMailer/PHPMailer/blob/master/examples/mail.phps

lib/linguist/languages.yml

@@ -2441,6 +2441,7 @@ PHP:
   - .php3
   - .php4
   - .php5
+  - .phps
   - .phpt
   filenames:
   - Phakefile