如何使用codeigniter更改mysql日期格式

I am developing a website using CI and mysql. i want to change date format from yyyy-mm-dd into dd-mm-yyyy. I know that i should use date_format function. But when I try to use it, it didn't work-the date format didn't change. Also i've try to add 2nd parameter(FALSE). but, I also found problem with that, especially, when i need to query more than 1 column. Here is my code:I am developing a website using CI and mysql. i