Made false, true, and null constants in PHP (#1694)

- This adds the `constant` alias to `false` and `true`.
- `null` is no longer `keyword` but a `constant`.

components/prism-php.js

@@ -12,8 +12,15 @@
  */
 (function (Prism) {
 	Prism.languages.php = Prism.languages.extend('clike', {
-		'keyword': /\b(?:__halt_compiler|abstract|and|array|as|break|callable|case|catch|class|clone|const|continue|declare|default|die|do|echo|else|elseif|empty|enddeclare|endfor|endforeach|endif|endswitch|endwhile|eval|exit|extends|final|finally|for|foreach|function|global|goto|if|implements|include|include_once|instanceof|insteadof|interface|isset|list|namespace|new|null|or|parent|print|private|protected|public|require|require_once|return|static|switch|throw|trait|try|unset|use|var|while|xor|yield)\b/i,
-		'constant': /\b[A-Z_][A-Z0-9_]*\b/,
+		'keyword': /\b(?:__halt_compiler|abstract|and|array|as|break|callable|case|catch|class|clone|const|continue|declare|default|die|do|echo|else|elseif|empty|enddeclare|endfor|endforeach|endif|endswitch|endwhile|eval|exit|extends|final|finally|for|foreach|function|global|goto|if|implements|include|include_once|instanceof|insteadof|interface|isset|list|namespace|new|or|parent|print|private|protected|public|require|require_once|return|static|switch|throw|trait|try|unset|use|var|while|xor|yield)\b/i,
+		'boolean': {
+			pattern: /\b(?:false|true)\b/i,
+			alias: 'constant'
+		},
+		'constant': [
+			/\b[A-Z_][A-Z0-9_]*\b/,
+			/\b(?:null)\b/i,
+		],
 		'comment': {
 			pattern: /(^|[^\\])(?:\/\*[\s\S]*?\*\/|\/\/.*)/,
 			lookbehind: true

tests/languages/php/boolean_feature.test

@@ -0,0 +1,17 @@
+FALSE
+false
+TRUE
+true
+
+----------------------------------------------------
+
+[
+	["boolean", "FALSE"],
+	["boolean", "false"],
+	["boolean", "TRUE"],
+	["boolean", "true"]
+]
+
+----------------------------------------------------
+
+Checks for booleans.

tests/languages/php/constant_feature.test

@@ -1,3 +1,4 @@
+null
 _
 X
 AZ
@@ -7,6 +8,7 @@ FOOBAR_42
 ----------------------------------------------------
 
 [
+	["constant", "null"],
 	["constant", "_"],
 	["constant", "X"],
 	["constant", "AZ"],

tests/languages/php/keyword_feature.test

@@ -46,7 +46,6 @@ isset
 list
 namespace;
 new;
-null
 or
 parent
 print
@@ -119,7 +118,6 @@ yield
 	["keyword", "list"],
 	["keyword", "namespace"], ["punctuation", ";"],
 	["keyword", "new"], ["punctuation", ";"],
-	["keyword", "null"],
 	["keyword", "or"],
 	["keyword", "parent"],
 	["keyword", "print"],