var total:Number=0;
btn.onPress=function()
{
    trace(total);

    var Phist:MovieClip = _root.attachMovie("phist", "phist"+(total++), _root.getNextHighestDepth());
    Phist._x = 100;
    Phist._y = 100;

    //or
    _root.attachMovie("phist", "phist"+(total++), _root.getNextHighestDepth());
    _root["phist"+total]._x = 100;
    _root["phist"+total]._y = 100;
}