I got some solution though it is not perfect answer.
The following is the example code to use 'dsLoad' function.
$returnFields = array('Email', 'FirstName', 'LastName');
$conDat = $app->dsLoad("Contact", 123, $returnFields);
dsLoad function is one which fetches required fields from database when we provide table name.
So, Use "Invoice" as table name and required fields as $returnFields array.
Here is the example code.
$returnFields = array('Id', 'ContactId', 'JobId');
$invoiceDetails = $app->dsLoad("Invoice", 123, $returnFields);