sort( $array ); 

foreach( $array AS $key=>$value ) {
  ( array_key_exists( (int)$key + 1 ) ) ? $next_playlist = (int)$key + 1 : $next_playlist = 0;
  $array_de_playlists[] = $key + "-" + $next_playlist;
}

print( "<pre>" );
var_dump( $array_de_playlist );
die;