"value1","value2","value3",...

I'm having difficulty figuring this out. I've tried several different methods, to no avail. Heres my code:

//saving record in a csv file
$file_name = "form.csv";
$first_row = "Process,VendorID,Type,Null1,FirstName,LastName,Null2\r\n";
$values = "Process,VendorID,Type,null,FirstName,LastName,null\r\n";
$is_first_row = false;
if(!file_exists($file_name))
{
$is_first_row = true ;
}
if (!$handle = fopen($file_name, 'a+')) {
die("Cannot open file ($file_name)");
exit;
}
if ($is_first_row)
{
if (fwrite($handle, $first_row ) === FALSE) {
die("Cannot write to file ($filename)");
exit;
}
}
if (fwrite($handle, $values) === FALSE) {
die("Cannot write to file ($filename)");
exit;
}
fclose($handle);