[php] what is the use of $this->uri->segment(3) in codeigniter pagination

By default the function returns FALSE (boolean) if the segment does not exist. There is an optional second parameter that permits you to set your own default value if the segment is missing. For example, this would tell the function to return the number zero in the event of failure: $product_id = $this->uri->segment(3, 0);

It helps avoid having to write code like this:

[if ($this->uri->segment(3) === FALSE)
{
    $product_id = 0;
}
else
{
    $product_id = $this->uri->segment(3);
}]