Converting Seconds and Duration to SRT (subtitle) time format (PHP)

I have an input like like this:

start: 10 | duration: 1 | text: Subtitle Text 1 
start: 15 | duration: 2 | text: Subtitle Text 2 
start: 20 | duration: 3 | text: Subtitle Text 3

It's a subtitle instruction set, that says following:

At 10 second of the video, show "Subtitle Text 1" for 1 seconds
At 15 second of the video, show "Subtitle Text 2" for 2 seconds
At 20 second of the video, show "Subtitle Text 3" for 3 seconds

This input needs to be converted into SRT format, so it becomes something like this:

1
00:00:10,000 --> 00:00:11,000
Subtitle Text 1

2
00:00:15,000 --> 00:00:17,000
Subtitle Text 2

3
00:00:20,000 --> 00:00:23,000
Subtitle Text 3

Would it be possible for someone to show me how would you go about converting any given seconds value into SRT format (00:00:00,000) by using PHP?

That's all I really need, the rest I can figure out myself.

Thanks, much appreciated.

private function seconds2SRT($seconds)
{
    $hours = 0;
    $whole = floor($seconds);
    $fraction = $seconds - $whole;
    $milliseconds = number_format($fraction, 3, '.', ',');
    $milliseconds_array = explode('.', strval($milliseconds));
    $milliseconds = $milliseconds_array[1];

    if ($seconds > 3600) {
        $hours = floor($seconds / 3600);
    }
    $seconds = $seconds % 3600;


    return str_pad($hours, 2, '0', STR_PAD_LEFT)
        . gmdate(':i:s', $seconds)
        . ($milliseconds ? ",$milliseconds" : '');
}