Why isn't this working with my metadata-string?

I have a page where I get the length of a mp3 file echoed to my page and it works fine.
I also want the length of the track to be inserted into my table, but the field is just empty.
How can I make this info into a string that I can insert into my table?

I guess there is something as simple as abc, but I just don’t get it…

$a = $m->get_metadata();
Track length: <?php echo $a['Length mm:ss']; ?> minutes
$sendlength=$a; ?>

The one I call $sendlength is the empty one. The rest is working fine.
If I output that one it just say Array

You’ll want to use $sendlength = $a[‘Length mm:ss’]; like you did for your echo statement.