How do I use prop. to only be applied to the element clicked?

Question

I am creating a payment form, part of which the user can select credit card or paypal. I am having issues with the display function. Should I be creating a new function for each element? This is what I have:

    <p>Billing Infomation</p>
    First Name: <input name="Billing_F_name" type="text" size="15" 
    value="Enter First Name"/> &nbsp;Last Name: <input name="Billing_L_name" 
    type="text" size="20" value="Enter Last Name"/><br />

    Method of Payment:<br /> 
    Credit Card <input name="payment" id="p1" type="radio" onclick="javascript: 
     payment()"/><br />

    Paypal <input name="payment" id="p4" type="radio" 
    onclick="javascript:payment()"/><br />
<script language="javascript" >
    function payment(click)
{
    if(document.getElementById("p1"){
  $('input:radio[name="payment1"][value="P_1"]').prop('checked=checked', true);

  document.write('<? $P_Method = 'P_1'; ?>');
   }

  else if(document.getElementsById("p4")){
  $('input:radio[name="payment4"][value="P_4"]').prop('checked=checked', true);}
  document.write('<? $P_Method = 'P_4'; ?>');
}
    </script>

BTW, just for your reference. The PHP set variables that are being set go to a php switch method, displaying their respective elements, which functions properly at the moment.

Answer 1

Pass the clicked element to the handler and use it.

javascript:payment(this)

..

function payment(el) {
    $(el).doSomething()
}

Answer 2

for execute in php you need use AJAX

<script language="javascript">
 function payment(dd)
{

if(dd.id=="p1") 
  document.write("<? $P_Method = 'P_1'; ?>");

else if(dd.id=="p4")
 document.write("<? $P_Method = 'P_4'; ?>");

}
</script>
<p>Billing Infomation</p>
First Name: <input name="Billing_F_name" size="15" value="Enter First Name" type="text"> 
&nbsp;Last Name: 
<input name="Billing_L_name" size="20" value="Enter Last Name" type="text"><br>

Method of Payment:<br> 
Credit Card <input name="payment" id="p1" onclick="payment(this)" type="radio"><br>

Paypal <input name="payment" id="p4" onclick="payment(this)" type="radio"><br>