$numbers = array(1,2,3,4,5,6,7,8,9,10,12);
$letters = array("un","deux","trois","quatre","cinq","six","sept","huit","neuf","dix","onze","douze");

$rand1 = rand(1,count($numbers));
$rand2 = rand(1,count($numbers));

$sign = ($rand1<=$rand2)?'+':(rand(0,3)?'-':'+');
$sign_display = ($sign=='-')?(rand(0,1)?'-':'moins'):(rand(0,1)?'+':'plus');

$int = $numbers[$rand1-1]+$numbers[$rand2-1]*(($sign=='-')?-1:1);

$challenge = (rand(0,1)?$numbers[$rand1-1]:$letters[$rand1-1]).' '.$sign_display.' '.(rand(0,1)?$numbers[$rand2-1]:$letters[$rand2-1]);
$salt = md5(uniqid());
$checksum = md5($int.$salt);

// display $challenge as a question and pass $salt and $checksum in hidden fields, then check that md5(answer.$salt)==$checksum