How to properly flush PHP echo output

I am using PHPMailer to send emails with attachments.

Each email takes about 5 seconds to send, and the PHP can send up to 5 emails with a total in that case of 10 PDFs...

During all that time, the browser shows a white page.

<?php
echo '<div>some spinner html</div>';
// code here to send emails
if (!$mail->send()) {
    echo 'Mailer Error: ' . $mail->ErrorInfo;
} else {
    echo 'Message sent!';
}

echo 'all the rest of the html';
?>

This doesn't work...
I have the feeling I can use an ob_start() to buffer the rest of the page and only show a spinner.

Real answer: Use JS and create an API using PHP

Best solution to your problem is using JavaScript and XHR and make a beautiful UI for it. If you want to show a loading and remove it after emails sent and show other stuff after disappearance of loading, it's only possible through JS. PHP is just a server side language capable of sending a response to browser and have no access to rendered page!

Flush output

Just use ob_flush() and flush() functions:

echo '<div>some spinner html</div>';
ob_flush();
flush();
// code here to send emails
if (!$mail->send()) {
    echo 'Mailer Error: ' . $mail->ErrorInfo;
    ob_flush();
    flush();
} else {
    echo 'Message sent!';
    ob_flush();
    flush();
}

echo 'all the rest of the html';

Using implicit flush

This looks like the previous one, but reduced a function call. this removes the flush() call!

ob_implicit_flush(true)
echo '<div>some spinner html</div>';
ob_flush();
// code here to send emails
if (!$mail->send()) {
    echo 'Mailer Error: ' . $mail->ErrorInfo;
    ob_flush();
} else {
    echo 'Message sent!';
    ob_flush();
}

echo 'all the rest of the html';

Turn off output buffer

Other solution is to turn off output buffer:

while (@ob_end_flush());
echo '<div>some spinner html</div>';
// code here to send emails
if (!$mail->send()) {
    echo 'Mailer Error: ' . $mail->ErrorInfo;
} else {
    echo 'Message sent!';
}

echo 'all the rest of the html';