$months = array(
   "Jan"=>1, "Feb"=>2, "Mar"=>3, "Apr"=>4, 
   "May"=>5, "Jun"=>6, "Jul"=>7, "Aug"=>8, 
   "Sep"=>9, "Oct"=>10, "Nov"=>11, "Dec"=>12
);
... is better than ...

$months["Jan"] = 1;
$months["Feb"] = 2;
$months["Mar"] = 3;
$months["Apr"] = 4;
$months["May"] = 5;
$months["Jun'] = 6;
$months["Jul"] = 7;
... etc. etc. etc. ...