This is a question you can read everywhere on the web with various answers:
$ext = end(explode(\'.\', $filename));
$ext = substr(strrchr($filename, \'.\'), 1
IMO, this is the best way if you have filenames like name.name.name.ext (ugly, but it sometimes happens):
$ext = explode('.', $filename); // Explode the string
$ext_len = count($ext) - 1; // Count the array -1 (because count() starts from 1)
$my_ext = $ext[$ext_len]; // Get the last entry of the array
echo $my_ext;