[Debug] workaround BC break in PHP 7.3

If the included file contains an error, it hides the real error. This
makes debugging harder.

How to reproduce:

```
composer create-project symfony/skeleton symfony-3.4 3.4
cd symfony-3.4
composer req monolog
```

Add to `monolog.yaml`:
```yaml
        elasticsearch:
            type: "elasticsearch"
            elasticsearch:
                host: 'elasticsearch'
                port: '9200'
            index: 'ep_php_logs_dev'
            level: 'debug'
            tags: 'monolog.logger'
            channels: ['!event']
```

This will fail because the the \Elastica\Client class does not exist.
But this error will be hidden by the `ClassNotFoundFatalErrorHandler`
because it will try to load the `Symfony\Component\Kernel\Client` and
this class extends `Symfony\Component\BrowserKit\Client`. The last one
is a soft dependency...

---

Before
```
Fatal error: Uncaught Error: Class 'Symfony\Component\BrowserKit\Client' not found in /tmp/symfony-3.4/vendor/symfony/http-kernel/Client.php:31
```

After:
```
Fatal error: Uncaught Symfony\Component\Debug\Exception\ClassNotFoundException: Attempted to load class "Client" from namespace "Elastica".
Did you forget a "use" statement for another namespace? in /tmp/symfony-es/var/cache/dev/ContainerWXN4mS9/srcApp_KernelDevDebugContainer.php:303
```

DebugClassLoader.php

@@ -168,7 +168,7 @@ class DebugClassLoader
 
     private function checkClass($class, $file = null)
     {
-        $exists = null === $file || \class_exists($class, false) || \interface_exists($class, false) || \trait_exists($class, false);
+        $exists = null === $file || class_exists($class, false) || interface_exists($class, false) || trait_exists($class, false);
 
         if (null !== $file && $class && '\\' === $class[0]) {
             $class = substr($class, 1);
@@ -186,13 +186,13 @@ class DebugClassLoader
             }
             $name = $refl->getName();
 
-            if ($name !== $class && 0 === \strcasecmp($name, $class)) {
+            if ($name !== $class && 0 === strcasecmp($name, $class)) {
                 throw new \RuntimeException(sprintf('Case mismatch between loaded and declared class names: "%s" vs "%s".', $class, $name));
             }
 
             $deprecations = $this->checkAnnotations($refl, $name);
 
-            if (isset(self::$php7Reserved[\strtolower($refl->getShortName())])) {
+            if (isset(self::$php7Reserved[strtolower($refl->getShortName())])) {
                 $deprecations[] = sprintf('The "%s" class uses the reserved name "%s", it will break on PHP 7 and higher', $name, $refl->getShortName());
             }
 
@@ -223,23 +223,23 @@ class DebugClassLoader
         $deprecations = [];
 
         // Don't trigger deprecations for classes in the same vendor
-        if (2 > $len = 1 + (\strpos($class, '\\') ?: \strpos($class, '_'))) {
+        if (2 > $len = 1 + (strpos($class, '\\') ?: strpos($class, '_'))) {
             $len = 0;
             $ns = '';
         } else {
-            $ns = \str_replace('_', '\\', \substr($class, 0, $len));
+            $ns = str_replace('_', '\\', substr($class, 0, $len));
         }
 
         // Detect annotations on the class
         if (false !== $doc = $refl->getDocComment()) {
             foreach (['final', 'deprecated', 'internal'] as $annotation) {
-                if (false !== \strpos($doc, $annotation) && preg_match('#\n\s+\* @'.$annotation.'(?:( .+?)\.?)?\r?\n\s+\*(?: @|/$|\r?\n)#s', $doc, $notice)) {
+                if (false !== strpos($doc, $annotation) && preg_match('#\n\s+\* @'.$annotation.'(?:( .+?)\.?)?\r?\n\s+\*(?: @|/$|\r?\n)#s', $doc, $notice)) {
                     self::${$annotation}[$class] = isset($notice[1]) ? preg_replace('#\.?\r?\n( \*)? *(?= |\r?\n|$)#', '', $notice[1]) : '';
                 }
             }
         }
 
-        $parent = \get_parent_class($class);
+        $parent = get_parent_class($class);
         $parentAndOwnInterfaces = $this->getOwnInterfaces($class, $parent);
         if ($parent) {
             $parentAndOwnInterfaces[$parent] = $parent;
@@ -254,22 +254,22 @@ class DebugClassLoader
         }
 
         // Detect if the parent is annotated
-        foreach ($parentAndOwnInterfaces + \class_uses($class, false) as $use) {
+        foreach ($parentAndOwnInterfaces + class_uses($class, false) as $use) {
             if (!isset(self::$checkedClasses[$use])) {
                 $this->checkClass($use);
             }
-            if (isset(self::$deprecated[$use]) && \strncmp($ns, \str_replace('_', '\\', $use), $len) && !isset(self::$deprecated[$class])) {
+            if (isset(self::$deprecated[$use]) && strncmp($ns, str_replace('_', '\\', $use), $len) && !isset(self::$deprecated[$class])) {
                 $type = class_exists($class, false) ? 'class' : (interface_exists($class, false) ? 'interface' : 'trait');
                 $verb = class_exists($use, false) || interface_exists($class, false) ? 'extends' : (interface_exists($use, false) ? 'implements' : 'uses');
 
                 $deprecations[] = sprintf('The "%s" %s %s "%s" that is deprecated%s.', $class, $type, $verb, $use, self::$deprecated[$use]);
             }
-            if (isset(self::$internal[$use]) && \strncmp($ns, \str_replace('_', '\\', $use), $len)) {
+            if (isset(self::$internal[$use]) && strncmp($ns, str_replace('_', '\\', $use), $len)) {
                 $deprecations[] = sprintf('The "%s" %s is considered internal%s. It may change without further notice. You should not use it from "%s".', $use, class_exists($use, false) ? 'class' : (interface_exists($use, false) ? 'interface' : 'trait'), self::$internal[$use], $class);
             }
         }
 
-        if (\trait_exists($class)) {
+        if (trait_exists($class)) {
             return $deprecations;
         }
 
@@ -296,7 +296,7 @@ class DebugClassLoader
 
             if (isset(self::$internalMethods[$class][$method->name])) {
                 list($declaringClass, $message) = self::$internalMethods[$class][$method->name];
-                if (\strncmp($ns, $declaringClass, $len)) {
+                if (strncmp($ns, $declaringClass, $len)) {
                     $deprecations[] = sprintf('The "%s::%s()" method is considered internal%s. It may change without further notice. You should not extend it from "%s".', $declaringClass, $method->name, $message, $class);
                 }
             }
@@ -307,7 +307,7 @@ class DebugClassLoader
             }
 
             foreach (['final', 'internal'] as $annotation) {
-                if (false !== \strpos($doc, $annotation) && preg_match('#\n\s+\* @'.$annotation.'(?:( .+?)\.?)?\r?\n\s+\*(?: @|/$|\r?\n)#s', $doc, $notice)) {
+                if (false !== strpos($doc, $annotation) && preg_match('#\n\s+\* @'.$annotation.'(?:( .+?)\.?)?\r?\n\s+\*(?: @|/$|\r?\n)#s', $doc, $notice)) {
                     $message = isset($notice[1]) ? preg_replace('#\.?\r?\n( \*)? *(?= |\r?\n|$)#', '', $notice[1]) : '';
                     self::${$annotation.'Methods'}[$class][$method->name] = [$class, $message];
                 }

ErrorHandler.php

@@ -382,6 +382,11 @@ class ErrorHandler
      */
     public function handleError($type, $message, $file, $line)
     {
+        // @deprecated to be removed in Symfony 5.0
+        if (\PHP_VERSION_ID >= 70300 && $message && '"' === $message[0] && 0 === strpos($message, '"continue') && preg_match('/^"continue(?: \d++)?" targeting switch is equivalent to "break(?: \d++)?"\. Did you mean to use "continue(?: \d++)?"\?$/', $message)) {
+            $type = E_DEPRECATED;
+        }
+
         // Level is the current error reporting level to manage silent error.
         $level = error_reporting();
         $silenced = 0 === ($level & $type);

FatalErrorHandler/ClassNotFoundFatalErrorHandler.php

@@ -185,7 +185,11 @@ class ClassNotFoundFatalErrorHandler implements FatalErrorHandlerInterface
             }
         }
 
-        require_once $file;
+        try {
+            require_once $file;
+        } catch (\Throwable $e) {
+            return null;
+        }
 
         foreach ($candidates as $candidate) {
             if ($this->classExists($candidate)) {

Tests/ErrorHandlerTest.php

@@ -234,7 +234,7 @@ class ErrorHandlerTest extends TestCase
             $logger
                 ->expects($this->once())
                 ->method('log')
-                ->will($this->returnCallback($warnArgCheck))
+                ->willReturnCallback($warnArgCheck)
             ;
 
             $handler = ErrorHandler::register();
@@ -262,7 +262,7 @@ class ErrorHandlerTest extends TestCase
             $logger
                 ->expects($this->once())
                 ->method('log')
-                ->will($this->returnCallback($logArgCheck))
+                ->willReturnCallback($logArgCheck)
             ;
 
             $handler = ErrorHandler::register();
@@ -318,7 +318,7 @@ class ErrorHandlerTest extends TestCase
         $logger
             ->expects($this->once())
             ->method('log')
-            ->will($this->returnCallback($logArgCheck))
+            ->willReturnCallback($logArgCheck)
         ;
 
         $handler = new ErrorHandler();
@@ -349,7 +349,7 @@ class ErrorHandlerTest extends TestCase
             $logger
                 ->expects($this->exactly(2))
                 ->method('log')
-                ->will($this->returnCallback($logArgCheck))
+                ->willReturnCallback($logArgCheck)
             ;
 
             $handler->setDefaultLogger($logger, E_ERROR);
@@ -503,7 +503,7 @@ class ErrorHandlerTest extends TestCase
             $logger
                 ->expects($this->once())
                 ->method('log')
-                ->will($this->returnCallback($logArgCheck))
+                ->willReturnCallback($logArgCheck)
             ;
 
             $handler->setDefaultLogger($logger, E_PARSE);