minor #7221 Changing field type in MySQL example (steveWinter)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7221).

Discussion
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Changing field type in MySQL example

It seems inconsistent that the MySQL table as the sess_id as VARBINARY, when the PostgeSQL and Microsoft SQL server use VARCHAR (which makes more sense to me)

Commits
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e772465 Changing field type in MySQL example

Author: xabbuh

doctrine/pdo_session_storage.rst

@@ -218,7 +218,7 @@ MySQL
 .. code-block:: sql
 
     CREATE TABLE `sessions` (
-        `sess_id` VARBINARY(128) NOT NULL PRIMARY KEY,
+        `sess_id` VARCHAR(128) NOT NULL PRIMARY KEY,
         `sess_data` BLOB NOT NULL,
         `sess_time` INTEGER UNSIGNED NOT NULL,
         `sess_lifetime` MEDIUMINT NOT NULL