Using FQ name for PHP_VERSION_ID

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #22307).

Discussion
----------

[Debug] Fix php notice

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Of course autoloading of an empty string should not actually happen (fixed that in https://github.com/twigphp/Twig/pull/2438) but if it does happen it should not throw a php notice.

```
Notice: Uninitialized string offset 0
```

Commits
-------

e333a1a [Debug] Fix php notice

DebugClassLoader.php

@@ -186,7 +186,7 @@ class DebugClassLoader
 
         $exists = class_exists($class, false) || interface_exists($class, false) || (function_exists('trait_exists') && trait_exists($class, false));
 
-        if ('\\' === $class[0]) {
+        if ($class && '\\' === $class[0]) {
             $class = substr($class, 1);
         }
 

ErrorHandler.php

@@ -101,6 +101,7 @@ class ErrorHandler
     private static $reservedMemory;
     private static $stackedErrors = array();
     private static $stackedErrorLevels = array();
+    private static $exitCode = 0;
 
     /**
      * Same init value as thrownErrors.
@@ -403,7 +404,7 @@ class ErrorHandler
                 $throw = new \ErrorException($this->levels[$type].': '.$message, 0, $type, $file, $line);
             }
 
-            if (PHP_VERSION_ID <= 50407 && (PHP_VERSION_ID >= 50400 || PHP_VERSION_ID <= 50317)) {
+            if (\PHP_VERSION_ID <= 50407 && (\PHP_VERSION_ID >= 50400 || \PHP_VERSION_ID <= 50317)) {
                 // Exceptions thrown from error handlers are sometimes not caught by the exception
                 // handler and shutdown handlers are bypassed before 5.4.8/5.3.18.
                 // We temporarily re-enable display_errors to prevent any blank page related to this bug.
@@ -477,6 +478,9 @@ class ErrorHandler
      */
     public function handleException($exception, array $error = null)
     {
+        if (null === $error) {
+            self::$exitCode = 255;
+        }
         if (!$exception instanceof \Exception) {
             $exception = new FatalThrowableError($exception);
         }
@@ -562,7 +566,7 @@ class ErrorHandler
             return;
         }
 
-        if (null === $error) {
+        if ($exit = null === $error) {
             $error = error_get_last();
         }
 
@@ -586,15 +590,21 @@ class ErrorHandler
             } else {
                 $exception = new FatalErrorException($handler->levels[$error['type']].': '.$error['message'], 0, $error['type'], $error['file'], $error['line'], 2, true, $trace);
             }
-        } elseif (!isset($exception)) {
-            return;
         }
 
         try {
-            $handler->handleException($exception, $error);
+            if (isset($exception)) {
+                self::$exitCode = 255;
+                $handler->handleException($exception, $error);
+            }
         } catch (FatalErrorException $e) {
             // Ignore this re-throw
         }
+
+        if ($exit && self::$exitCode) {
+            $exitCode = self::$exitCode;
+            register_shutdown_function('register_shutdown_function', function () use ($exitCode) { exit($exitCode); });
+        }
     }
 
     /**

ExceptionHandler.php

@@ -434,7 +434,7 @@ EOF;
     {
         @trigger_error('The '.__METHOD__.' method is deprecated since version 2.7 and will be removed in 3.0.', E_USER_DEPRECATED);
 
-        return htmlspecialchars($str, ENT_QUOTES | (PHP_VERSION_ID >= 50400 ? ENT_SUBSTITUTE : 0), 'UTF-8');
+        return htmlspecialchars($str, ENT_QUOTES | (\PHP_VERSION_ID >= 50400 ? ENT_SUBSTITUTE : 0), 'UTF-8');
     }
 
     /**
@@ -442,7 +442,7 @@ EOF;
      */
     private function escapeHtml($str)
     {
-        return htmlspecialchars($str, ENT_QUOTES | (PHP_VERSION_ID >= 50400 ? ENT_SUBSTITUTE : 0), $this->charset);
+        return htmlspecialchars($str, ENT_QUOTES | (\PHP_VERSION_ID >= 50400 ? ENT_SUBSTITUTE : 0), $this->charset);
     }
 
     /**

Tests/DebugClassLoaderTest.php

@@ -11,10 +11,11 @@
 
 namespace Symfony\Component\Debug\Tests;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Debug\DebugClassLoader;
 use Symfony\Component\Debug\ErrorHandler;
 
-class DebugClassLoaderTest extends \PHPUnit_Framework_TestCase
+class DebugClassLoaderTest extends TestCase
 {
     /**
      * @var int Error reporting level before running tests
@@ -60,7 +61,7 @@ class DebugClassLoaderTest extends \PHPUnit_Framework_TestCase
 
     public function testUnsilencing()
     {
-        if (PHP_VERSION_ID >= 70000) {
+        if (\PHP_VERSION_ID >= 70000) {
             $this->markTestSkipped('PHP7 throws exceptions, unsilencing is not required anymore.');
         }
         if (defined('HHVM_VERSION')) {
@@ -110,7 +111,7 @@ class DebugClassLoaderTest extends \PHPUnit_Framework_TestCase
             restore_error_handler();
             restore_exception_handler();
             $this->assertStringStartsWith(__FILE__, $exception->getFile());
-            if (PHP_VERSION_ID < 70000) {
+            if (\PHP_VERSION_ID < 70000) {
                 $this->assertRegExp('/^Runtime Notice: Declaration/', $exception->getMessage());
                 $this->assertEquals(E_STRICT, $exception->getSeverity());
             } else {
@@ -226,7 +227,7 @@ class DebugClassLoaderTest extends \PHPUnit_Framework_TestCase
 
     public function testReservedForPhp7()
     {
-        if (PHP_VERSION_ID >= 70000) {
+        if (\PHP_VERSION_ID >= 70000) {
             $this->markTestSkipped('PHP7 already prevents using reserved names.');
         }
 

Tests/ErrorHandlerTest.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Debug\Tests;
 
+use PHPUnit\Framework\TestCase;
 use Psr\Log\LogLevel;
 use Symfony\Component\Debug\ErrorHandler;
 use Symfony\Component\Debug\Exception\ContextErrorException;
@@ -21,7 +22,7 @@ use Symfony\Component\Debug\Exception\ContextErrorException;
  * @author Robert Schönthal <seroscho@googlemail.com>
  * @author Nicolas Grekas <p@tchwork.com>
  */
-class ErrorHandlerTest extends \PHPUnit_Framework_TestCase
+class ErrorHandlerTest extends TestCase
 {
     public function testRegister()
     {

Tests/Exception/FlattenExceptionTest.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Debug\Tests\Exception;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Debug\Exception\FlattenException;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException;
@@ -27,7 +28,7 @@ use Symfony\Component\HttpKernel\Exception\ServiceUnavailableHttpException;
 use Symfony\Component\HttpKernel\Exception\TooManyRequestsHttpException;
 use Symfony\Component\HttpKernel\Exception\UnsupportedMediaTypeHttpException;
 
-class FlattenExceptionTest extends \PHPUnit_Framework_TestCase
+class FlattenExceptionTest extends TestCase
 {
     public function testStatusCode()
     {

Tests/ExceptionHandlerTest.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Debug\Tests;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Debug\ExceptionHandler;
 use Symfony\Component\Debug\Exception\OutOfMemoryException;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
@@ -18,7 +19,7 @@ use Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException;
 
 require_once __DIR__.'/HeaderMock.php';
 
-class ExceptionHandlerTest extends \PHPUnit_Framework_TestCase
+class ExceptionHandlerTest extends TestCase
 {
     protected function setUp()
     {

Tests/FatalErrorHandler/ClassNotFoundFatalErrorHandlerTest.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Debug\Tests\FatalErrorHandler;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\ClassLoader\ClassLoader as SymfonyClassLoader;
 use Symfony\Component\ClassLoader\UniversalClassLoader as SymfonyUniversalClassLoader;
 use Symfony\Component\Debug\Exception\FatalErrorException;
@@ -18,7 +19,7 @@ use Symfony\Component\Debug\FatalErrorHandler\ClassNotFoundFatalErrorHandler;
 use Symfony\Component\Debug\DebugClassLoader;
 use Composer\Autoload\ClassLoader as ComposerClassLoader;
 
-class ClassNotFoundFatalErrorHandlerTest extends \PHPUnit_Framework_TestCase
+class ClassNotFoundFatalErrorHandlerTest extends TestCase
 {
     public static function setUpBeforeClass()
     {

Tests/FatalErrorHandler/UndefinedFunctionFatalErrorHandlerTest.php

@@ -11,10 +11,11 @@
 
 namespace Symfony\Component\Debug\Tests\FatalErrorHandler;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Debug\Exception\FatalErrorException;
 use Symfony\Component\Debug\FatalErrorHandler\UndefinedFunctionFatalErrorHandler;
 
-class UndefinedFunctionFatalErrorHandlerTest extends \PHPUnit_Framework_TestCase
+class UndefinedFunctionFatalErrorHandlerTest extends TestCase
 {
     /**
      * @dataProvider provideUndefinedFunctionData

Tests/FatalErrorHandler/UndefinedMethodFatalErrorHandlerTest.php

@@ -11,10 +11,11 @@
 
 namespace Symfony\Component\Debug\Tests\FatalErrorHandler;
 
+use PHPUnit\Framework\TestCase;
 use Symfony\Component\Debug\Exception\FatalErrorException;
 use Symfony\Component\Debug\FatalErrorHandler\UndefinedMethodFatalErrorHandler;
 
-class UndefinedMethodFatalErrorHandlerTest extends \PHPUnit_Framework_TestCase
+class UndefinedMethodFatalErrorHandlerTest extends TestCase
 {
     /**
      * @dataProvider provideUndefinedMethodData

composer.json

@@ -24,7 +24,7 @@
     },
     "require-dev": {
         "symfony/class-loader": "~2.2",
-        "symfony/http-kernel": "~2.3.24|~2.5.9|~2.6,>=2.6.2"
+        "symfony/http-kernel": "~2.3.24|~2.5.9|^2.6.2"
     },
     "autoload": {
         "psr-4": { "Symfony\\Component\\Debug\\": "" },

phpunit.xml.dist

@@ -5,6 +5,8 @@
          backupGlobals="false"
          colors="true"
          bootstrap="vendor/autoload.php"
+         failOnRisky="true"
+         failOnWarning="true"
 >
     <php>
         <ini name="error_reporting" value="-1" />