MakiMaki, that won't work. You can only use the isin operator to make one single comparison.

Scakk's suggestion makes the recommended approach. You can also combine them with the && operator:

if ($numeric == 318) && ($numeric == 312) { halt }

You can, however, use the $count():

if ($count($numeric,312,318)) { halt }

in place of the isin, but isin is not considered appropriate.