Made `flymake-phpcs-standard' more safe.

`flymake-phpcs-standard' is only safe as a local variable if the
specified standard is not pointing to a file/path to a standard.

So the safe-local-variable property is adjusted to only be safe when
the value is string-or-null-p AND it is not file-exists-p.

This might fail in the case where you specify i.e. PEAR as standard and
also have a file or folder named PEAR relative to the buffer. PHPCS will
choose the installed standard but `flymake-phpcs-standard' will consider
the setting unsafe. At least it only fails to the safe side - it won't
allow unsafe values but can mark a few safe values as unsafe.

Another approach would be to only accept all installed standards
(phpcs -i) as safe values but we'll spare the overhead of running
external commands.

README.mkdn

@@ -28,7 +28,7 @@ something like the following to your .emacs:
 (setq flymake-phpcs-command "~/projects/emacs-flymake-phpcs/bin/flymake_phpcs")
 
 ;; Customize the coding standard checked by phpcs
-(setq flymake-phpcs-standard
+(set-default 'flymake-phpcs-standard
   "~/projects/devtools/php_codesniffer/MyCompanyStandard")
 
 ;; Show the name of sniffs in warnings (eg show

flymake-phpcs.el

@@ -1,6 +1,6 @@
 ;;; flymake-phpcs.el --- Flymake handler for PHP to invoke PHP-CodeSniffer
 ;;
-;; Copyright (C) 2011-2012  Free Software Foundation, Inc.
+;; Copyright (C) 2011-2012, 2014  Free Software Foundation, Inc.
 ;;
 ;; Author: Sam Graham <libflymake-phpcs-emacs BLAHBLAH illusori.co.uk>
 ;; Maintainer: Sam Graham <libflymake-phpcs-emacs BLAHBLAH illusori.co.uk>
@@ -42,6 +42,9 @@
 (defcustom flymake-phpcs-standard "PEAR"
   "The coding standard to pass to phpcs via --standard."
   :group 'flymake-phpcs
+  :safe (lambda (value)
+          (and (string-or-null-p value)
+               (not (file-exists-p value))))
   :type 'string)
 
 (defcustom flymake-phpcs-show-rule nil