文件导出脚本挂起在大输出

下面是我用来导出MySQL数据库为。csv文件的脚本。

对于较小的输出(大约30,000行以下)，它似乎做得很好，但是挂在较大的输出上，并且无法完成(总共100万行)。

知道为什么吗?

此外，PHPMyAdmin导出实用程序转储它的速度非常快(只需几秒钟)。想知道它是怎么做到的。

if(isset($_POST['submit'])) {
// Fetch Record from Database
$output = "";
$table = "tt_output";
$sql = mysqli_query($connection,"select * from {$table}");
$columns_total = mysqli_num_fields($sql);
// Get The Field Name
$query = "SHOW COLUMNS FROM {$table}"; $result_set = mysqli_query($connection,$query);
while ($result = mysqli_fetch_array($result_set)) {
    $heading = $result[0];
    $output .= trim($heading.',');
}
$output = substr($output,0,strlen($output)-1)."'r'n";
// Get Records from the table
while ($row = mysqli_fetch_array($sql)) {
    for ($i = 0; $i < $columns_total; $i++) {
        $output .='"'.$row["$i"].'",';
    }
    $output = substr($output,0,strlen($output)-1)."'r'n";
}
// Download the file
$filename = "output".".csv";
header("Pragma: public", true);
header("Expires: 0"); // set expiration time
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Content-Type: application/force-download");
header("Content-Type: application/octet-stream");
header("Content-Type: application/download");
header('Content-Disposition: attachment; filename='.$filename);
header("Content-Transfer-Encoding: binary");
header('Connection: Keep-Alive');
echo $output;
exit;

}

...
header('Connection: Keep-Alive');
// Get Records from the table
while ($row = mysqli_fetch_array($sql)) {
    for ($i = 0; $i < $columns_total; $i++) {
        $output .='"'.$row["$i"].'",';
    }
    echo substr($output,0,strlen($output)-1)."'r'n";
}