readfile downloads an empty file and wrong file type

I am using the code below to download a csv file from a button click.

JS:

$(document).on("click", "#CSV", function () {

    var csv_value = $('#result').table2CSV({
        delivery: 'value'
    });
    console.log(csv_value);
    $.ajax({
        type: "POST",
        url: "csv.php",
        data: {
            csv: csv_value
        },
        success: function (html) {
             window.location.href = "download.php";
        }
    });

});

csv.php:

require_once ("session_start.php");
if (isset ( $_POST ['csv'] )) {
$file = $_POST['csv'];
$_SESSION ['csvf'] = $file; 
} else {
    echo "No CSV Data";
}

download.php:

session_start();
    $file =$_SESSION['csvf'];
    $filename = $file."_".date("Y-m-d_H-i",time());
    header ( "Content-type: application/vnd.ms-excel" );
    header ( "Content-disposition: filename=" . $filename . ".csv" );
    readfile($filename);
    exit ();

When I execute the function, the file downloaded is download.php despite the correct headers being passed (confirmed via console), the file is also empty. Also when I use print() it prints the CSV data so I know the file isn’t empty but it also generates a .php file and prints all the contents inside including the headers. How do I fix this issue?

Edit

I tried brute forcing the download using the following code in .htaccess to no avail.

<FilesMatch ".(?i:csv)$">
  ForceType application/octet-stream
  Header set Content-Disposition attachment
</FilesMatch>

The result is still a download.php file.

Answer

The below code worked for me using print() instead of readfile()

session_start ();
$file = $_SESSION ['csvf'];
$_table = $_SESSION['table']; //table Name
$filename =$_table."_".date ( "Y-m-d_H-i", time () ).".csv";  
header ( "Content-type: text/csv" );
header ( "Content-Disposition: attachment; filename=".$filename );
header ( 'Expires: 0' );
header ( 'Cache-Control: must-revalidate' );
header ( 'Pragma: public' );
header ( 'Content-Length: ' . filesize ( $file ) );
print ($file) ;
exit ();