电子邮件表单写入.xlsx NOT .xls

Question

I have an email form that has a few variables and I need that information to write to a .xlsx file.

I have never written to that kind of file before but have written to a .xls file hundreds of times.

The code below does not work, it will create the file but nothing is written to it.

Can I please have some feedback on how to get this structure to write to a .xlsx file?

$Course = $_POST["Course"];
$CourseNumber = $_POST["CourseNumber"];
$MemberFee = $_POST["MemberFee"];
$Email = $_POST["Email"];

$db.="$Course\t";
$db.="$CourseNumber\t";
$db.="$$MemberFee\t";
$db.="$Email\t";
$db.="
";

$dbout=fopen("tls-member.xlsx","a+");
fwrite($dbout, $db);
fwrite($dbout,$message);
fclose($dbout);