INSERT IGNORE using Codeigniter

As I also encountered a similar problem, I finally chose a little bit more "elegant" solution like the one below. A complete insert_batch query that is something that uses Rocket's answer AND transactions:

$this->db->trans_start();
foreach ($items as $item) {
       $insert_query = $this->db->insert_string('table_name', $item);
       $insert_query = str_replace('INSERT INTO', 'INSERT IGNORE INTO', $insert_query);
       $this->db->query($insert_query);
    }
$this->db->trans_complete();

That will also wrap everything on a transaction resulting on a faster query like doing a insert_batch(). Well not as fast as insert_batch() but faster than a single query for each entry of course. I hope it will help someone.

For batch uploads you might need something more like:

foreach ($data as $data_item) {
    $insert_query = $this->db->insert_string('my_table', $data_item);
    $insert_query = str_replace('INSERT INTO', 'INSERT IGNORE INTO', $insert_query);
    $this->db->query($insert_query);
}

UPDATE: Use dcostalis's version (in the comments below this one), it will scale much better :-)

This is basically a modification of Rocket Hazmat's suggestion, which is great, but doesn't take into account the fact that str_replace operates on the entire string and might inadvertently affect the data.

$insert_query = $this->db->insert_string('my_table', $data);
$insert_query = preg_replace('/INSERT INTO/','INSERT IGNORE INTO',$insert_query,1);
$this->db->query($insert_query);

Don't use insert_batch, as it actually runs the query. You want insert_string

$insert_query = $this->db->insert_string('my_table', $data);
$insert_query = str_replace('INSERT INTO','INSERT IGNORE INTO',$insert_query);
$this->db->query($insert_query);

UPDATE: This doesn't work for batch queries, only one row at a time.