PHP Mode for Emacs Code

Commit rest of changes intended for 2008-04-12 fixes.

Make grammatical throughout document.
(Introduction): Add description of intended audience.
(Editing): Document mark-defun, narrow-to-defun and widen.
(Contributors): Add Aaron Iba.

Insert various comments for maintaining the
manual.

Update modification date to today's date.

(php-font-lock-keywords-1, php-font-lock-keywords-2): Add [^_$] to
beginning and [^_] to end of font-lock regular expressions to
avoid false matches in user variable and function names. Affects
highlighting of self, parent, print, break, case, continue, other
keywords and internal constants.

Add missing paren from last commit.

(php-mode-modified): Use a string literal rather than a computed
value.

(php-tags, php-tags-key): Move definition to above
php-mode to avoid byte compiler warning. Use eval-when-compile
for regexp-opt.
(php-font-lock-keywords-1): Use font-lock-preprocessor-face for
PHP and ASP tags.

(php-mode): Overwrite C-style macros rule to recognize PHP tags
instead.
(php-tags, php-tags-key): New constants.
(php-font-lock-keywords-1): Use php-tags-key for syntax
highlighting of PHP tags.

(php-mode): Add commented-out c-lang-defvar statements below
commented-out c-lang-defconst.