Convert XML RSS Feed Audio Length from Bytes to Minutes/Seconds in PHP

I am scraping an RSS Feed which includes <enclosure> elements for audio files. According to the spec, the length is given in bytes.

 <enclosure url="https://www.w3schools.com/media/audio-file.m4a" length="67960845" type="audio/x-m4a" />

I need to convert this length into an actual measurable time (i.e. hours, minutes, seconds). This would require knowing the bitrate (I believe), which I happen to have! The RSS feed comes from Anchor and according to Anchor, they sample their audio in stereo at 44.1 kHz, 128 kbit/s CBR.

I know the provided example length="67960845" converts to 71 minutes. I tried to reverse engineer it but I can't really find a good formula in order to reliably convert this like:

<?php 
    $length = 67960845;
    $seconds = $length / ????; 
?>

This request is in PHP but could apply to any language scraping an RSS feed.

Solution

If you convert the length to bits you can then divide it by the bitrate and then get the number of seconds:

(67960845*8)/128/1000/60 = 70.792546875

So...

$length = 67960845;
$bitrate_kbps = 128;
$seconds = ceil(($length * 8) / $bitrate_kbps / 1000 / 60);

Does this work for you?