Combine an unknown number of .mp3 files

Combine an unknown number of .mp3 files

I have this:
$number = 1;
$mp3 = file_get_contents("http://example.com/text=".$value);
$file = md5($value)."-".$number.".mp3";
file_put_contents($file, $mp3);
$number++;
Result: create abcdef-1.mp3, abcdef-2.mp3, abcdef-3.mp3.
Now I want to combine abcdef-1.mp3 with abcdef-2.mp3 and abcdef-3.mp3.
This works:
file_put_contents('abcdef.mp3',
file_get_contents('abcdef-1.mp3') .
file_get_contents('abcdef-2.mp3') .
file_get_contents('abcdef-3.mp3'));
But is not suitable for my code, because abcdef differ. Numbers too.
I've tried:
$number = 1;
$mp3 = file_get_contents("http://example.com/text=".$value);
$file = md5($value)."-".$number.".mp3";
file_put_contents($file, $mp3);
file_put_contents(md5($value).".mp3", file_get_contents($file));
$number++;
Result: create abcdef-1.mp3, abcdef-2.mp3, abcdef-3.mp3 and abcdef.mp3 but
when I play it's only abcdef-1.mp3. Where is the problem?