function changeImg(obj){
  var n = obj.value;
  document.getElementById("ph_7").style.display="none";
  document.getElementById("ph_31").style.display="none";
  document.getElementById("ph_92").style.display="none";
  document.getElementById("ph_183").style.display="none";
  if(n == 7){
  	document.getElementById("ph_7").style.display="block";
  }
  if(n == 31){
  	document.getElementById("ph_31").style.display="block";
  }
  if(n == 92){
  	document.getElementById("ph_92").style.display="block";
  }
  if(n == 183){
  	document.getElementById("ph_183").style.display="block";
  }
}