Store date only in mysql with php codeigniter

What is your reason for doing this? I can't see any reasonable use for it.

You cannot change the way MySQL stores dates, no. But you can of course format them when reading/writing them. You can do it in SQL query like this:

For example you can use STR_TO_DATE function to format the date when inserting it:

INSERT INTO mytable (mydate) VALUES (STR_TO_DATE('12/31/2009', '%m/%d/%Y'))

And vice versa:

SELECT DATE_FORMAT(mydate, '%m/%d/%Y') FROM mytable /* returns 12/31/2009 */

But as FactalizeR pointed out, it is not a good practice to do it in the query and it should be moved to script, like this (considering you are using PHP).

$date = '12/31/2009';
$date = date('Y-m-d', strtotime($date));
mysql_query("INSERT INTO mytable (mydate) VALUES ({$date})");

And vice versa

$date = mysql_result(mysql_query("SELECT mydate FROM mytable"), 0, 0);
$date = date('m/d/Y', strtotime($date)); //returns 12/31/2009

$this->db->set($s['dbcolumns'][$i], date('Y-m-d', strtotime($s['value'][$i])));

To store time

$this->db->set($s['dbcolumns'][$i], date('H:i:s', strtotime($s['value'][$i])));