How can I seek in a video outputted by PHP?

For outputting a big video (duration 20m46s, size 1.34 GB),
I use the following code (file video.php):

<?php
define('CHUNK_SIZE', 1024*1024); // Size (in bytes) of tiles chunk
// Read a file and display its content chunk by chunk
function readfile_chunked($filename, $retbytes = TRUE) {
    $buffer = '';
    $cnt    = 0;
    $handle = fopen($filename, 'rb');

    if ($handle === false) {
        return false;
    }

    while (!feof($handle)) {
        $buffer = fread($handle, CHUNK_SIZE);
        echo $buffer;
        ob_flush();
        flush();

        if ($retbytes) {
            $cnt += strlen($buffer);
        }
    }

    $status = fclose($handle);

    if ($retbytes && $status) {
        return $cnt; // return num. bytes delivered like readfile() does.
    }

    return $status;
}

$filename = 'video.mp4';
$mimetype = 'video/mp4';
header('Content-Type: '.$mimetype );
readfile_chunked($filename);

?>

The video is watched in a HTML page with the following code:

<video controls>

<source src='video.php' type='video/mp4'>
  
</video>

However, seeking in the video doesn’t work correctly.
For example, if I move the play cursor for seeking to the 18:00 timestamp, it will
instead seek to the 0:58 timestamp.

How can I solve this?