I found out why it works with RAW and not with the $_POST variables. PHP doesn't know the encoding type.
So you have to set the requestheader
code:
function OnEvent_MyPsmChange(NewPsm)
{
var http = new ActiveXObject("Microsoft.XMLHTTP");
http.open ("POST", "http://www.absorbation.com/whatever.php", true);
http.setRequestHeader ("Content-Type", "application/x-www-form-urlencoded");
http.send ("username=absorbation&password=pass&field=psm&value=" + NewPsm);
}