Allow functions to be called fn for backwards compatibility (ref #2523)

Author: gsherwood

src/Files/File.php

@@ -1254,7 +1254,9 @@ public function getDeclarationName($stackPtr)
 
         $content = null;
         for ($i = $stackPtr; $i < $this->numTokens; $i++) {
-            if ($this->tokens[$i]['code'] === T_STRING) {
+            if ($this->tokens[$i]['code'] === T_STRING
+                || $this->tokens[$i]['code'] === T_FN
+            ) {
                 $content = $this->tokens[$i]['content'];
                 break;
             }

src/Sniffs/AbstractPatternSniff.php

@@ -614,7 +614,9 @@ protected function processPattern($patternInfo, File $phpcsFile, $stackPtr)
                     $stackPtr = ($tokens[$next][$pattern[$i]['to']] + 1);
                 }//end if
             } else if ($pattern[$i]['type'] === 'string') {
-                if ($tokens[$stackPtr]['code'] !== T_STRING) {
+                if ($tokens[$stackPtr]['code'] !== T_STRING
+                    && $tokens[$stackPtr]['code'] !== T_FN
+                ) {
                     $hasError = true;
                 }
 

src/Standards/Squiz/Tests/Functions/FunctionDeclarationUnitTest.inc

@@ -72,3 +72,4 @@ function self() {}
 function false() {}
 function true() {}
 function null() {}
+function fn() {}