The following can be used to run the refresh command in PHP 7.2 as ldap_exop_refresh comes with 7.3.
$dn= 'cn=user,dc=test,dc=org';
$ttl = 3600;
$data = '';
$data .= '80' . sprintf("%'.02x", strlen($dn)) . bin2hex($dn);
$data .= '81' . sprintf("%'.02x", 4) . sprintf("%'.08x", $ttl);
$data = hex2bin('30' . sprintf("%'.02x", strlen($data)/2) . $data);
$success = ldap_exop($ldapHandle, LDAP_EXOP_REFRESH, $data);