yii\db\mysql\QueryBuilder::createIndex PHP Method

    public function createIndex($name, $table, $columns, $unique = false)
    {
        return 'ALTER TABLE ' . $this->db->quoteTableName($table) . ($unique ? ' ADD UNIQUE INDEX ' : ' ADD INDEX ') . $this->db->quoteTableName($name) . ' (' . $this->buildColumns($columns) . ')';
    }