Problem with concatenation of variable

Within your foreach , collect in an array all the IDs, and in the output of foreach execute the implode() function of PHP to transform into a string with the interspersed separator you need:

$ids = array();

foreach(????) {
    $ids[] = $this->session->userdata('last_id');
}
echo implode('|', $ids);

Read PHP's implode function.

If in the variable $this->session->userdata('last_id') returns an array with the ids you want, just do:

$ids = implode('|', $this->session->userdata('last_id'));

And when I echo the variable ids will return exactly what you wanted. Hope this helps! ;)

You can concatenate the string as @rray put in the comments:

$ids_ .= $id_inscrito .'|';

Remembering that you must initialize the $ids_ variable before the loop. Another point to consider is that the last id will always be followed by a | character. A more complete solution would look like this:

$ids_ = '';
foreach (...) {
    // codigo

    // se o $ids_ não estiver vazio adiciona '|' antes do próximo id
    $ids_ .= ($ids_ == '') ? $id_inscrito : '|' . $id_inscrito ;
}

Another possible way is to use arrays, as @HaroldoTorres put in your answer:

$ids_ = array();
foreach (...) {
    // codigo

    $ids_[] = $id_inscrito;
}
$ids_ = implode('|', $ids_);