Codeigniter transactions with multiple functions

Yes. Think about code execution. Just because code execution goes in and out of functions doesn't mean that the DB class isn't going to handle what it's told to do. Functions/methods do limit variable scope, help organize your code, and have other benefits. Functions/methods aren't going to interfere with your transactions.

I have performed the tests but if an insert fails all queries are executed and does not undo the queries that have to be undone in order to maintain the integrity of the data.

PHP Code:

<?php

public function init_invoice()
{
    $products = self::products_related_invoice();
    $total = '0';

    if(!$products) {
        return;
    }

    foreach ($products as $value) {
        $total = $total + $value['price'];
    }

    $this->db->trans_start();
    $id_invoice             = self::add_new_invoice($total);
    $add_invoice_details    = self::add_invoice_details($id_invoice, $products);

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

public function add_new_invoice($total)
{
   $data = array(            
        'id_user'           => $this->session->user_id,
        'id_invoice_status' => '1', 
        'total' => $total, 
        'auto_create' => 1 
    );
    $this->db->insert('bill', $data);
    if($this->db->affected_rows()) {
        $id_invoice = $this->db->insert_id();       
        return $id_invoice;
    }
    return false;
}

public function add_invoice_details($id_invoice, $productos)
{
    /**
     * Default var
     */
    $status = false;

    foreach ($productos as $key => $value) {
        $query = $this->db->simple_query('
            INSERT INTO '.$this->db->dbprefix('bill_details').'
                   (`id_invoice`,`id_product`,`created_by`,`modified_by`) 
            VALUES (
                '.$this->db->escape($id_invoice).',
                '.$this->db->escape($value['id']).' ,
                '.$this->db->escape($this->session->user_id).'
                '.$this->db->escape($this->session->user_id).'
            )
        ');

        if(!$query) {
            $status = true;
        }
    }

    if($status == false) {
        return true;
    }

    return false;
}

public function products_related_invoice()
{
    $query = $this->db->select('id, price')
                      ->from('products')
                      ->where('status', '1') 
                      ->where('related_invoice', '1') 
                      ->get();
    if($query->num_rows() > 0) {
        
        $data = $query->result_array();           
        return $data;
    }
    return false;
}