If you want to use substr(), you can easily add the dashes or slashes like this..
$datetime = "20130409163705";
$yyyy = substr($datetime,0,4);
$mm = substr($datetime,4,6);
$dd = substr($datetime,6,8);
$hh = substr($datetime,8,10);
$MM = substr($datetime,10,12);
$ss = substr($datetime,12,14);
$dt_formatted = $mm."/".$dd."/".$yyyy." ".$hh.":".$MM.":".$ss;
You can figure out any further formatting from that point.