AbstractPlatform::_getTransactionIsolationLevelSQL() must return a string

lib/Doctrine/DBAL/Platforms/SQLAnywherePlatform.php

@@ -1293,13 +1293,13 @@ SQL
     {
         switch ($level) {
             case TransactionIsolationLevel::READ_UNCOMMITTED:
-                return 0;
+                return '0';
             case TransactionIsolationLevel::READ_COMMITTED:
-                return 1;
+                return '1';
             case TransactionIsolationLevel::REPEATABLE_READ:
-                return 2;
+                return '2';
             case TransactionIsolationLevel::SERIALIZABLE:
-                return 3;
+                return '3';
             default:
                 throw new InvalidArgumentException('Invalid isolation level:' . $level);
         }

lib/Doctrine/DBAL/Platforms/SqlitePlatform.php

@@ -168,11 +168,11 @@ class SqlitePlatform extends AbstractPlatform
     {
         switch ($level) {
             case TransactionIsolationLevel::READ_UNCOMMITTED:
-                return 0;
+                return '0';
             case TransactionIsolationLevel::READ_COMMITTED:
             case TransactionIsolationLevel::REPEATABLE_READ:
             case TransactionIsolationLevel::SERIALIZABLE:
-                return 1;
+                return '1';
             default:
                 return parent::_getTransactionIsolationLevelSQL($level);
         }