Improve recognition of constants in PHP (#1688)

The pattern for PHP constants now have to be a valid identifier and recognizes single-letter names.

Author: volado

components/prism-php.js

@@ -13,7 +13,7 @@
 (function (Prism) {
 	Prism.languages.php = Prism.languages.extend('clike', {
 		'keyword': /\b(?:and|or|xor|array|as|break|case|cfunction|class|const|continue|declare|default|die|do|else|elseif|enddeclare|endfor|endforeach|endif|endswitch|endwhile|extends|for|foreach|function|include|include_once|global|if|new|return|static|switch|use|require|require_once|var|while|abstract|interface|public|implements|private|protected|parent|throw|null|echo|print|trait|namespace|final|yield|goto|instanceof|finally|try|catch)\b/i,
-		'constant': /\b[A-Z0-9_]{2,}\b/,
+		'constant': /\b[A-Z_][A-Z0-9_]*\b/,
 		'comment': {
 			pattern: /(^|[^\\])(?:\/\*[\s\S]*?\*\/|\/\/.*)/,
 			lookbehind: true

components/prism-php.min.js

@@ -1,10 +1,14 @@
+_
+X
 AZ
 PRISM
 FOOBAR_42
 
 ----------------------------------------------------
 
 [
+	["constant", "_"],
+	["constant", "X"],
 	["constant", "AZ"],
 	["constant", "PRISM"],
 	["constant", "FOOBAR_42"]