Fix heredoc pattern and support 7.3 heredoc

This change has support for 7.3 form of heredoc and suppression of
errors occurring in that code.  In this new syntax, arbitrary space is
allowed before the termination ID of heredoc.

https://wiki.php.net/rfc/flexible_heredoc_nowdoc_indentation
http://php.net/manual/en/migration73.new-features.php

php-mode.el

@@ -983,8 +983,9 @@ this ^ lineup"
 (defun php-heredoc-end-re (heredoc-start)
   "Build a regular expression for the end of a heredoc started by the string HEREDOC-START."
   ;; Extract just the identifier without <<< and quotes.
-  (string-match "\\_<.+?\\_>" heredoc-start)
-  (concat "^\\(" (match-string 0 heredoc-start) "\\)\\W"))
+  (save-excursion
+    (string-match "\\_<.+?\\_>" heredoc-start)
+    (concat "^\\s-*\\(" (match-string 0 heredoc-start) "\\)\\W")))
 
 (defun php-syntax-propertize-function (start end)
   "Apply propertize rules from START to END."

tests/issue-439.php

@@ -31,3 +31,13 @@
 $g = <<<"いろは"
 Let'go Justin
 いろは;
+
+var_dump(<<<"ABC"
+Let'go Justin
+ABC);
+
+if (1 === 1) {
+    var_dump(<<<"ABC"
+    Let'go Justin
+    ABC);
+}