Restore FnTokenEmulator (#1141)

As of PHP 7.4, functions can no longer be named fn, due to the new arrow
function syntax. Consequently, pre-7.4, functions may be called fn. This
pull request restores a previously present token emulator that made this
behavior possible.

Fixes #1139.

Author: WoutervdBrink

lib/PhpParser/Lexer/Emulative.php

@@ -9,6 +9,7 @@
 use PhpParser\Lexer\TokenEmulator\AttributeEmulator;
 use PhpParser\Lexer\TokenEmulator\EnumTokenEmulator;
 use PhpParser\Lexer\TokenEmulator\ExplicitOctalEmulator;
+use PhpParser\Lexer\TokenEmulator\FnTokenEmulator;
 use PhpParser\Lexer\TokenEmulator\MatchTokenEmulator;
 use PhpParser\Lexer\TokenEmulator\NullsafeTokenEmulator;
 use PhpParser\Lexer\TokenEmulator\PipeOperatorEmulator;
@@ -40,6 +41,7 @@ public function __construct(?PhpVersion $phpVersion = null) {
         $this->hostPhpVersion = PhpVersion::getHostVersion();
 
         $emulators = [
+            new FnTokenEmulator(),
             new MatchTokenEmulator(),
             new NullsafeTokenEmulator(),
             new AttributeEmulator(),

lib/PhpParser/Lexer/TokenEmulator/FnTokenEmulator.php

@@ -0,0 +1,20 @@
+<?php declare(strict_types=1);
+
+namespace PhpParser\Lexer\TokenEmulator;
+
+use PhpParser\PhpVersion;
+
+// Retained for reverse emulation support only.
+final class FnTokenEmulator extends KeywordEmulator {
+    public function getPhpVersion(): PhpVersion {
+        return PhpVersion::fromString('7.4');
+    }
+
+    public function getKeywordString(): string {
+        return 'fn';
+    }
+
+    public function getKeywordToken(): int {
+        return \T_FN;
+    }
+}

test/code/parser/stmt/function/fn.test

@@ -0,0 +1,21 @@
+Function with the name fn
+-----
+<?php
+function fn() {}
+-----
+!!version=7.3
+array(
+    0: Stmt_Function(
+        attrGroups: array(
+        )
+        byRef: false
+        name: Identifier(
+            name: fn
+        )
+        params: array(
+        )
+        returnType: null
+        stmts: array(
+        )
+    )
+)