Use single quotes for string literal

Author: chadicus

src/Codeception/Lib/Driver/MySql.php

@@ -35,7 +35,7 @@ public function getPrimaryKey($tableName)
         if (!isset($this->primaryKeys[$tableName])) {
             $primaryKey = [];
             $stmt = $this->getDbh()->query(
-                'SHOW KEYS FROM ' . $this->getQuotedName($tableName) . ' WHERE Key_name = "PRIMARY"'
+                'SHOW KEYS FROM ' . $this->getQuotedName($tableName) . " WHERE Key_name = 'PRIMARY'"
             );
             $columns = $stmt->fetchAll(\PDO::FETCH_ASSOC);