Content update changes in PHP Coding Standards (#65)

* Update wordpress-coding-standards/php.md

Textual update of the PHP coding standards document.

Includes:
* Removing outdated references to PEAR, BackPress and the defunct Hackers mailinglist.
* Updating various texts to be in line with the current state of PHP.
* Removed unnecessary "PHP" H2 subheader.
* Changed the rest of the headers from H3 to H2.
* Removed the credits and changelog as they were wildly inaccurate and out of date.
* Various small textual clarifications and grammar improvements.

* Update wordpress-coding-standards/php.md

Co-authored-by: Gary Jones <github@garyjones.io>

* Update wordpress-coding-standards/php.md

Co-authored-by: Gary Jones <github@garyjones.io>

* Update wordpress-coding-standards/php.md

Co-authored-by: Gary Jones <github@garyjones.io>

* Add suggestions from the review

* Add suggestions from the review

* Remove unnecessary line and reorder the paragraphs

Put the inline documentation above the WPCS mention.

* Update wordpress-coding-standards/php.md

Co-authored-by: Sergey Biryukov <sergeybiryukov.ru@gmail.com>

Co-authored-by: Gary Jones <github@garyjones.io>
Co-authored-by: Stephen Edgar <stephen@netweb.com.au>
Co-authored-by: Sergey Biryukov <sergeybiryukov.ru@gmail.com>

Author: 4 people