bug symfony#25922 [HttpFoundation] Use the correct syntax for session gc based on Pdo driver (tanasecosminromeo)

This PR was submitted for the master branch but it was squashed and merged into the 2.7 branch instead (closes symfony#25922).

Discussion
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[HttpFoundation] Use the correct syntax for session gc based on Pdo driver

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#25665 symfony#24456
| License       | MIT
| Doc PR        |

The initial fix for symfony#24456 was wrong, since it only accounted for Postgres. @WhiteEagle88 correctly identified symfony#25665 - but having time as SIGNED is ... off, since time shouldn't be negative. Using CAST to solve this issue surely has a performance penalty, so I believe the best approach is to have a switch based on the driver.

Commits
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826dfbd [HttpFoundation] Use the correct syntax for session gc based on Pdo driver

Author: fabpot

src/Symfony/Component/HttpFoundation/Session/Storage/Handler/PdoSessionHandler.php

@@ -375,7 +375,11 @@ public function close()
             $this->gcCalled = false;
 
             // delete the session records that have expired
-            $sql = "DELETE FROM $this->table WHERE $this->lifetimeCol < :time - $this->timeCol";
+            if ('mysql' === $this->driver) {
+                $sql = "DELETE FROM $this->table WHERE $this->lifetimeCol + $this->timeCol < :time";
+            } else {
+                $sql = "DELETE FROM $this->table WHERE $this->lifetimeCol < :time - $this->timeCol";
+            }
 
             $stmt = $this->pdo->prepare($sql);
             $stmt->bindValue(':time', time(), \PDO::PARAM_INT);