$PT = array(
  '107-18-6' => 'B',
  '7784-34-1' => 'B',
  '353-42-4' => 'B',
  '4170-30-3' => 'B',
  '123-73-9' => 'B',
  '108-91-8' => 'B',
  '106-89-8' => 'B',
  '107-15-3' => 'B',
  '78-82-0' => 'B',
  '60-34-4' => 'B',
  '556-64-9' => 'B',
  '79-21-0' => 'B',
  '594-42-3' => 'B',
  '110-89-4' => 'B',
  '107-12-0' => 'B',
  '109-61-5' => 'B',
  '509-14-8' => 'B',
  '7550-45-0' => 'B'
);
$pt = $PT[[!substance_cas_no]];
return $pt ? $pt : NULL;

See U.S. EPA (1999), Exhibit B-2 (Appendix B, page B-4).