Deborah, yes, you can do this modifying generated inlude\..._functions.phpfile.
Open it with a text editor and locate GetDefaultValuefunction. Then modify it this way, please see my changes in bold:
if($field=="RadioField") return "Value1";
if($field=="CheckboxField") return 1;
50038011/10/2005
Hello, Thanks you for your response. I'm working on radio buttons and I think I found a bug: When I use 'lookup table', my display field take the value of my link field. By modifing the following lines in source/functions.php l580, it's run:
Deborah, thank you for finding this bug. We'll correct it in the next PHPRunner update. Table based Radio buttons are rarely used so we missed the error.