Assign XML obtained data as default value

You need to check what value $veri->Currency[0]->BanknoteBuying holds. Try to print it on the page instead of assigning to session variable.

I have checked and it is working well, I have seen the currency showing as in XML

How exactly you checking this?
What you see with your eyes in XML file and what value $veri->Currency[0]->BanknoteBuying variable has in PHP code are two different things.

Well, I have tried to print as follow on List Page:Before process event with following code

In the above code echo $dolartl; is showing the currency value as it is but echo $_SESSION["UsdTrl"]; part displays nothing.
However, if I use same code onAdd Page: Before Process event the web page giving complete error and not opening

$data[$dolartl] is completely meaningless here. Where this $data is coming from?

It is what I have found from topics as "how to assign value to session variable"
I will appreciate if you just lead me with a correct code, instead of criticizing my mistakes. I am learning and sorry to bother you

Now Okey, with following code I can see the values on the page

The only thing can you tell me why it is working on the List page Before Process event, but not working on Add Page: Before Display event? when I add the code on add page, the page is not opening at all!!
The Error message comes with very long list and at the end of page: Fatal error: Uncaught exception 'Exception' with message 'Serialization of 'SimpleXMLElement' is not allowed' in [no active file]:0 Stack trace: #0 {main} thrown in [no active file] on line 0

Now All is working well with following code, thanks for your time and effort...