diff --git a/composer.json b/composer.json
index 781e376722d..34a604eb0eb 100644
--- a/composer.json
+++ b/composer.json
@@ -47,7 +47,7 @@
     },
     "suggest": {
         "ext-dom": "For handling output formats in XML",
-        "ext-mbstring": "For handling non-UTF8 characters in cache signature.",
+        "ext-mbstring": "For handling non-UTF8 characters.",
         "php-cs-fixer/phpunit-constraint-isidenticalstring": "For IsIdenticalString constraint.",
         "php-cs-fixer/phpunit-constraint-xmlmatchesxsd": "For XmlMatchesXsd constraint.",
         "symfony/polyfill-mbstring": "When enabling `ext-mbstring` is not possible."
diff --git a/src/Fixer/ClassNotation/FinalInternalClassFixer.php b/src/Fixer/ClassNotation/FinalInternalClassFixer.php
index b8503d43ac3..39d66aa8ddb 100644
--- a/src/Fixer/ClassNotation/FinalInternalClassFixer.php
+++ b/src/Fixer/ClassNotation/FinalInternalClassFixer.php
@@ -72,12 +72,12 @@ public function getDefinition()
     /**
      * {@inheritdoc}
      *
-     * Must run before FinalStaticAccessFixer, SelfStaticAccessorFixer.
+     * Must run before FinalStaticAccessFixer, ProtectedToPrivateFixer, SelfStaticAccessorFixer.
      * Must run after PhpUnitInternalClassFixer.
      */
     public function getPriority()
     {
-        return 0;
+        return 67;
     }
 
     /**
diff --git a/src/Fixer/ClassNotation/ProtectedToPrivateFixer.php b/src/Fixer/ClassNotation/ProtectedToPrivateFixer.php
index 2b6760dd131..6c6c502b7ff 100644
--- a/src/Fixer/ClassNotation/ProtectedToPrivateFixer.php
+++ b/src/Fixer/ClassNotation/ProtectedToPrivateFixer.php
@@ -53,6 +53,7 @@ protected function test()
      * {@inheritdoc}
      *
      * Must run before OrderedClassElementsFixer.
+     * Must run after FinalInternalClassFixer.
      */
     public function getPriority()
     {
diff --git a/src/Fixer/Comment/NoEmptyCommentFixer.php b/src/Fixer/Comment/NoEmptyCommentFixer.php
index 68c2950e731..652daab5be6 100644
--- a/src/Fixer/Comment/NoEmptyCommentFixer.php
+++ b/src/Fixer/Comment/NoEmptyCommentFixer.php
@@ -157,7 +157,7 @@ private function isEmptyComment($content)
     {
         static $mapper = [
             self::TYPE_HASH => '|^#\s*$|', // single line comment starting with '#'
-            self::TYPE_SLASH_ASTERISK => '|^/\*\s*\*/$|', // comment starting with '/*' and ending with '*/' (but not a PHPDoc)
+            self::TYPE_SLASH_ASTERISK => '|^/\*[\s\*]*\*+/$|', // comment starting with '/*' and ending with '*/' (but not a PHPDoc)
             self::TYPE_DOUBLE_SLASH => '|^//\s*$|', // single line comment starting with '//'
         ];
 
diff --git a/src/Fixer/FunctionNotation/PhpdocToReturnTypeFixer.php b/src/Fixer/FunctionNotation/PhpdocToReturnTypeFixer.php
index d49e290d63c..dda3fa27d69 100644
--- a/src/Fixer/FunctionNotation/PhpdocToReturnTypeFixer.php
+++ b/src/Fixer/FunctionNotation/PhpdocToReturnTypeFixer.php
@@ -75,6 +75,11 @@ final class PhpdocToReturnTypeFixer extends AbstractFixer implements Configurati
      */
     private $classRegex = '/^\\\\?[a-zA-Z_\\x7f-\\xff](?:\\\\?[a-zA-Z0-9_\\x7f-\\xff]+)*(?<array>\[\])*$/';
 
+    /**
+     * @var array<string, bool>
+     */
+    private $returnTypeCache = [];
+
     /**
      * {@inheritdoc}
      */
@@ -251,6 +256,10 @@ protected function applyFix(\SplFileInfo $file, Tokens $tokens)
                 continue;
             }
 
+            if (!$this->isValidType($returnType)) {
+                continue;
+            }
+
             $this->fixFunctionDefinition($tokens, $startIndex, $isNullable, $returnType);
         }
     }
@@ -337,4 +346,23 @@ private function findReturnAnnotations(Tokens $tokens, $index)
 
         return $doc->getAnnotationsOfType('return');
     }
+
+    /**
+     * @param string $returnType
+     *
+     * @return bool
+     */
+    private function isValidType($returnType)
+    {
+        if (!\array_key_exists($returnType, $this->returnTypeCache)) {
+            try {
+                Tokens::fromCode(sprintf('<?php function f():%s {}', $returnType));
+                $this->returnTypeCache[$returnType] = true;
+            } catch (\ParseError $e) {
+                $this->returnTypeCache[$returnType] = false;
+            }
+        }
+
+        return $this->returnTypeCache[$returnType];
+    }
 }
diff --git a/src/Fixer/PhpUnit/PhpUnitInternalClassFixer.php b/src/Fixer/PhpUnit/PhpUnitInternalClassFixer.php
index 013cec41043..bb0bf276b0a 100644
--- a/src/Fixer/PhpUnit/PhpUnitInternalClassFixer.php
+++ b/src/Fixer/PhpUnit/PhpUnitInternalClassFixer.php
@@ -49,7 +49,7 @@ public function getDefinition()
      */
     public function getPriority()
     {
-        return 1;
+        return 68;
     }
 
     /**
diff --git a/src/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixer.php b/src/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixer.php
index 77760a42362..82816f192f8 100644
--- a/src/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixer.php
+++ b/src/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixer.php
@@ -114,6 +114,10 @@ protected function applyFix(\SplFileInfo $file, Tokens $tokens)
                 $content = Preg::replaceCallback(
                     '/^(\s*\*\s*@\w+\s+'.$optionalTypeRegEx.')(\p{Lu}?(?=\p{Ll}|\p{Zs}))(.*)$/',
                     static function (array $matches) {
+                        if (\function_exists('mb_strtolower')) {
+                            return $matches[1].mb_strtolower($matches[2]).$matches[3];
+                        }
+
                         return $matches[1].strtolower($matches[2]).$matches[3];
                     },
                     $startLine->getContent(),
diff --git a/src/Fixer/Whitespace/BlankLineBeforeStatementFixer.php b/src/Fixer/Whitespace/BlankLineBeforeStatementFixer.php
index d732f8b9a9a..6783a1e6d75 100644
--- a/src/Fixer/Whitespace/BlankLineBeforeStatementFixer.php
+++ b/src/Fixer/Whitespace/BlankLineBeforeStatementFixer.php
@@ -76,6 +76,8 @@ public function configure(array $configuration = null)
         foreach ($this->configuration['statements'] as $key) {
             $this->fixTokenMap[$key] = self::$tokenMap[$key];
         }
+
+        $this->fixTokenMap = array_values($this->fixTokenMap);
     }
 
     /**
@@ -263,7 +265,7 @@ public function getPriority()
      */
     public function isCandidate(Tokens $tokens)
     {
-        return $tokens->isAnyTokenKindsFound(array_values($this->fixTokenMap));
+        return $tokens->isAnyTokenKindsFound($this->fixTokenMap);
     }
 
     /**
@@ -271,13 +273,16 @@ public function isCandidate(Tokens $tokens)
      */
     protected function applyFix(\SplFileInfo $file, Tokens $tokens)
     {
-        $tokenKinds = array_values($this->fixTokenMap);
         $analyzer = new TokensAnalyzer($tokens);
 
         for ($index = $tokens->count() - 1; $index > 0; --$index) {
             $token = $tokens[$index];
 
-            if (!$token->isGivenKind($tokenKinds) || ($token->isGivenKind(T_WHILE) && $analyzer->isWhilePartOfDoWhile($index))) {
+            if (!$token->isGivenKind($this->fixTokenMap)) {
+                continue;
+            }
+
+            if ($token->isGivenKind(T_WHILE) && $analyzer->isWhilePartOfDoWhile($index)) {
                 continue;
             }
 
@@ -331,7 +336,7 @@ private function shouldAddBlankLine(Tokens $tokens, $prevNonWhitespace)
                     continue;
                 }
 
-                return !$tokens[$j]->equals('{');
+                return $tokens[$j]->equalsAny([';', '}']);
             }
         }
 
diff --git a/tests/AutoReview/FixerFactoryTest.php b/tests/AutoReview/FixerFactoryTest.php
index 82d4391dea7..4162b9d0d55 100644
--- a/tests/AutoReview/FixerFactoryTest.php
+++ b/tests/AutoReview/FixerFactoryTest.php
@@ -95,6 +95,7 @@ public function provideFixersPriorityCases()
             [$fixers['escape_implicit_backslashes'], $fixers['single_quote']],
             [$fixers['explicit_string_variable'], $fixers['simple_to_complex_string_variable']],
             [$fixers['final_internal_class'], $fixers['final_static_access']],
+            [$fixers['final_internal_class'], $fixers['protected_to_private']],
             [$fixers['final_internal_class'], $fixers['self_static_accessor']],
             [$fixers['fully_qualified_strict_types'], $fixers['no_superfluous_phpdoc_tags']],
             [$fixers['function_to_constant'], $fixers['native_function_casing']],
diff --git a/tests/AutoReview/ProjectCodeTest.php b/tests/AutoReview/ProjectCodeTest.php
index 13c6722555d..29ad10894e6 100644
--- a/tests/AutoReview/ProjectCodeTest.php
+++ b/tests/AutoReview/ProjectCodeTest.php
@@ -12,11 +12,8 @@
 
 namespace PhpCsFixer\Tests\AutoReview;
 
-if (!class_exists(\PHPUnit\Runner\Version::class)) {
-    class_alias('PHPUnit_Runner_Version', \PHPUnit\Runner\Version::class);
-}
-
 use PhpCsFixer\DocBlock\DocBlock;
+use PhpCsFixer\Event\Event;
 use PhpCsFixer\Preg;
 use PhpCsFixer\Tests\TestCase;
 use PhpCsFixer\Tokenizer\Token;
@@ -416,6 +413,67 @@ static function (\ReflectionMethod $reflectionMethod) use ($reflectionClass) {
         }
     }
 
+    /**
+     * @dataProvider provideSrcClassCases
+     * @dataProvider provideTestClassCases
+     *
+     * @param string $className
+     */
+    public function testAllCodeContainSingleClassy($className)
+    {
+        $headerTypes = [
+            T_ABSTRACT,
+            T_AS,
+            T_COMMENT,
+            T_DECLARE,
+            T_DOC_COMMENT,
+            T_FINAL,
+            T_LNUMBER,
+            T_NAMESPACE,
+            T_NS_SEPARATOR,
+            T_OPEN_TAG,
+            T_STRING,
+            T_USE,
+            T_WHITESPACE,
+        ];
+
+        $rc = new \ReflectionClass($className);
+        $file = $rc->getFileName();
+        $tokens = Tokens::fromCode(file_get_contents($file));
+        $isEvent = Event::class === $rc->getName(); // remove this exception when no longer needed
+        $classyIndex = null;
+
+        static::assertTrue($tokens->isAnyTokenKindsFound(Token::getClassyTokenKinds()), sprintf('File "%s" should contains a classy.', $file));
+
+        foreach ($tokens as $index => $token) {
+            if ($token->isClassy()) {
+                $classyIndex = $index;
+
+                break;
+            }
+
+            if (!$token->isGivenKind($headerTypes) && !$token->equalsAny([';', '=', '(', ')']) && !$isEvent) {
+                static::fail(sprintf('File "%s" should only contains single classy, found "%s" @ %d.', $file, $token->toJson(), $index));
+            }
+        }
+
+        static::assertNotNull($classyIndex, sprintf('File "%s" does not contain a classy.', $file));
+
+        $nextTokenOfKind = $tokens->getNextTokenOfKind($classyIndex, ['{']);
+
+        if (!\is_int($nextTokenOfKind)) {
+            throw new \UnexpectedValueException('Classy without {} - braces.');
+        }
+
+        $classyEndIndex = $tokens->findBlockEnd(Tokens::BLOCK_TYPE_CURLY_BRACE, $nextTokenOfKind);
+
+        if ($isEvent) {
+            static::assertNotNull($tokens->getNextNonWhitespace($classyEndIndex), sprintf('File "%s" should not only contains a single classy.', $file));
+        } else {
+            static::assertNull($tokens->getNextNonWhitespace($classyEndIndex), sprintf('File "%s" should only contains a single classy.', $file));
+        }
+    }
+
     public function provideSrcClassCases()
     {
         return array_map(
diff --git a/tests/Fixer/Comment/NoEmptyCommentFixerTest.php b/tests/Fixer/Comment/NoEmptyCommentFixerTest.php
index b12161b0baa..9cd2eb97510 100644
--- a/tests/Fixer/Comment/NoEmptyCommentFixerTest.php
+++ b/tests/Fixer/Comment/NoEmptyCommentFixerTest.php
@@ -212,6 +212,26 @@ public function provideFixCases()
                     $bar = 2;
                 ',
             ],
+            [
+                '<?php
+                    '.'
+                ',
+                '<?php
+                    /*
+                     *
+                     */
+                ',
+            ],
+            [
+                '<?php
+                    '.'
+                ',
+                '<?php
+                    /********
+                     *
+                     ********/
+                ',
+            ],
         ];
     }
 
diff --git a/tests/Fixer/FunctionNotation/PhpdocToReturnTypeFixerTest.php b/tests/Fixer/FunctionNotation/PhpdocToReturnTypeFixerTest.php
index ced56c34ec2..c95a4a49acb 100644
--- a/tests/Fixer/FunctionNotation/PhpdocToReturnTypeFixerTest.php
+++ b/tests/Fixer/FunctionNotation/PhpdocToReturnTypeFixerTest.php
@@ -60,6 +60,15 @@ public function provideFixCases()
             'invalid class 2' => [
                 '<?php /** @return \\Foo\\\\Bar */ function my_foo() {}',
             ],
+            'invalid class 3' => [
+                '<?php /** @return Break */ function my_foo() {}',
+            ],
+            'invalid class 4' => [
+                '<?php /** @return __CLASS__ */ function my_foo() {}',
+            ],
+            'invalid class 5' => [
+                '<?php /** @return I\Want\To\Break\Free */ function queen() {}',
+            ],
             'blacklisted class methods' => [
                 '<?php
 
diff --git a/tests/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixerTest.php b/tests/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixerTest.php
index 2cb5dbf75a0..3b13a7a9323 100644
--- a/tests/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixerTest.php
+++ b/tests/Fixer/Phpdoc/PhpdocAnnotationWithoutDotFixerTest.php
@@ -207,6 +207,20 @@ function foo($str) {}',
      */
     function nothingToDo() {}',
             ],
+            [
+                '<?php
+/**
+ * @param string $bar τάχιστη
+ */
+function foo ($bar) {}
+',
+                '<?php
+/**
+ * @param string $bar Τάχιστη.
+ */
+function foo ($bar) {}
+',
+            ],
         ];
     }
 }
diff --git a/tests/Fixer/Whitespace/BlankLineBeforeStatementFixerTest.php b/tests/Fixer/Whitespace/BlankLineBeforeStatementFixerTest.php
index c089cdcdf39..09f483743ae 100644
--- a/tests/Fixer/Whitespace/BlankLineBeforeStatementFixerTest.php
+++ b/tests/Fixer/Whitespace/BlankLineBeforeStatementFixerTest.php
@@ -1019,6 +1019,17 @@ function foo()
     // comment
 
     return "bar";
+}',
+            ],
+            [
+                '<?php
+
+function foo()
+{
+    switch ($foo) {
+        case 2: // comment
+            return 1;
+    }
 }',
             ],
         ];
diff --git a/tests/Fixtures/Integration/priority/final_internal_class,protected_to_private.test b/tests/Fixtures/Integration/priority/final_internal_class,protected_to_private.test
new file mode 100644
index 00000000000..d2c52160ad4
--- /dev/null
+++ b/tests/Fixtures/Integration/priority/final_internal_class,protected_to_private.test
@@ -0,0 +1,25 @@
+--TEST--
+Integration of fixers: final_internal_class,protected_to_private.
+--RULESET--
+{"final_internal_class": true, "protected_to_private": true}
+--EXPECT--
+<?php
+/**
+ * @internal
+ */
+final class Foo
+{
+    private $bar;
+    private function baz() {}
+}
+
+--INPUT--
+<?php
+/**
+ * @internal
+ */
+class Foo
+{
+    protected $bar;
+    protected function baz() {}
+}