How to use simplexml_load_string in PHP?

See PHP: simplexml_load_string - Manual[^]

Quote:

Takes a well-formed XML string and returns it as an object.

The error messages indicates that the argument is not a well-formed XML string or even NULL.

Further help would require to know what types are returned by $client->GetInstalmentDetails() and $iResponse->GetResult, and how the value of the last looks like (the XML string).

I would expect that GetResult is a function and not a value (using parentheses). You should also add error checks where necessary and possible:

$iResponse = $client->GetInstalmentDetails(array("PID" => $proId));
// Should check $iResponse for failure here

// Using function call here: $iResponse->GetResult()
$XmlString = $iResponse->GetResult();
// Should check for NULL string here
$GetData = simplexml_load_string($XmlString);