Modify syntax-entry $ to "_" from "'"

In practice, $ is just part of the variable name, which is more
advantageous than considering it as “Expression prefixes”.

If you want to be compatible with older syntax tables, add the
following code to your own .emacs file (init.el):

    (with-eval-after-load "php-mode"
      (modify-syntax-entry ?$  "'"   php-mode-syntax-table))

Past discussions are in the following issues:
#320
#564

Author: zonuexe

Diff for: php-mode.el

@@ -1073,7 +1073,7 @@ After setting the stylevars run hooks according to STYLENAME
     (modify-syntax-entry ?\" "\""  table)
     (modify-syntax-entry ?#  "< b" table)
     (modify-syntax-entry ?\n "> b" table)
-    (modify-syntax-entry ?$  "'"   table)
+    (modify-syntax-entry ?$  "_"   table)
     table))
 
 ;;;###autoload