how to get last insert id after insert query in codeigniter active record

Question

Try this

function add_post($post_data){
   $this->db->insert('posts', $post_data);
   $insert_id = $this->db->insert_id();

   return  $insert_id;
}

In case of multiple inserts you could use

$this->db->trans_start();
$this->db->trans_complete();

A transaction isn't needed here, this should suffice:

function add_post($post_data) {
    $this->db->insert('posts',$post_data);
    return $this->db->insert_id();
}

$id = $this->db->insert_id();

From the documentation:

$this->db->insert_id()

The insert ID number when performing database inserts.

Therefore, you could use something like this:

$lastid = $this->db->insert_id();

Using the mysqli PHP driver, you can't get the insert_id after you commit.

The real solution is this:

function add_post($post_data){
  $this->db->trans_begin();
  $this->db->insert('posts',$post_data);

  $item_id = $this->db->insert_id();

  if( $this->db->trans_status() === FALSE )
  {
    $this->db->trans_rollback();
    return( 0 );
  }
  else
  {
    $this->db->trans_commit();
    return( $item_id );
  }
}

Source for code structure: https://codeigniter.com/user_guide/database/transactions.html#running-transactions-manually