bug #15282 [HttpFoundation] Behaviour change in PHP7 for substr (Nicofuma)

This PR was merged into the 2.3 branch.

Discussion
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[HttpFoundation] Behaviour change in PHP7 for substr

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

In PHP7 the behaviour of substr() changed.
To resume: "Truncating an entire string should result in a string."

See: https://bugs.php.net/bug.php?id=62922

Commits
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ba6000b [HttpFoundation] Behaviour change in PHP7 for substr

src/Symfony/Component/HttpFoundation/Request.php

@@ -1790,7 +1790,8 @@ class Request
             $requestUri = substr($requestUri, 0, $pos);
         }
 
-        if (null !== $baseUrl && false === $pathInfo = substr($requestUri, strlen($baseUrl))) {
+        $pathInfo = substr($requestUri, strlen($baseUrl));
+        if (null !== $baseUrl && (false === $pathInfo || '' === $pathInfo)) {
             // If substr() returns false then PATH_INFO is set to an empty string
             return '/';
         } elseif (null === $baseUrl) {