PHP: Download MySQL table

Question

I want to download a MySQL table as CSV file but I am getting this error "Allowed memory size of 1342177280 bytes exhausted". I guess that is because the file is first created and then downloaded. How can I achive that the user can start downloading the file from 0, in this case I think that I wouldn't need so much memory.

This is my code so far:

$output         = "";
$table          = "export_table";
$sql            = mysql_query("select * from $table");
$columns_total  = mysql_num_fields($sql);

// Get The Field Name
for ($i = 0; $i < $columns_total; $i++) {
    $heading    =   mysql_field_name($sql, $i);
    $output     .= '"'.$heading.'",';
}
$output .="\n";
// Get Records from the table
while ($row = mysql_fetch_array($sql)) {
for ($i = 0; $i < $columns_total; $i++) {
$output .='"'.$row["$i"].'",';
}
$output .="\n";
}
// Download the file
$filename =  "export.csv";
header('Content-type: application/csv');
header('Content-Disposition: attachment; filename='.$filename);
echo $output;

Answer 1

So at the end this is what did what I wanted:

$db = mysqli_connect("localhost", "user", "password", "database");

$result = mysqli_query($db, "SELECT * FROM export_table", MYSQLI_USE_RESULT);

header("Content-Type: application/force-download");
header("Content-Type: application/octet-stream");
header("Content-Type: application/download");
header("Content-Disposition: attachment;filename=\"export_table.csv\"");
header("Content-Transfer-Encoding: binary");
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: max-age=0, no-cache, must-revalidate, proxy-revalidate, post-check=0, pre-check=0");
header("Cache-Control: private",false);

$output = fopen('php://output', 'w');

    fputcsv($output, array('ID','Column1','Column2','Column3'));

while ($row = mysqli_fetch_assoc($result))
    {
    fputcsv($output, $row);
    }

fclose($output);
mysqli_free_result($result);
mysqli_close($db);