bug #17964 [FrameworkBundle] Fix PhpDocExtractor registration (dunglas)

This PR was merged into the 3.1-dev branch.

Discussion
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[FrameworkBundle] Fix PhpDocExtractor registration

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

Fix for the fix... (#17931). `class_exists` on an interface will never work. I've tested this one in real condition and it works.

Commits
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89467b5 [FrameworkBundle] Fix PhpDocExtractor registration

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -1010,7 +1010,7 @@ private function registerPropertyInfoConfiguration(array $config, ContainerBuild
 
         $loader->load('property_info.xml');
 
-        if (class_exists('phpDocumentor\Reflection\DocBlockFactoryInterface')) {
+        if (interface_exists('phpDocumentor\Reflection\DocBlockFactoryInterface')) {
             $definition = $container->register('property_info.php_doc_extractor', 'Symfony\Component\PropertyInfo\Extractor\PhpDocExtractor');
             $definition->addTag('property_info.description_extractor', array('priority' => -1000));
             $definition->addTag('property_info.type_extractor', array('priority' => -1001));