How to get a file’s extension from the filename

This function gets the extension of a provided filename. It is useful in upload scripts in order to permit the upload of only certain types of files.

function get_extension ($filename) {

   // This function reads the extension of a file. It is used in 
   // the image uploading process.

   // Find the position of the LAST dot in the filename. 
   $dot_position = strrpos($filename,".");

   // Check to see if we got results above, if not, return nothing,
   // ending the function.

   if (!$dot_position) {
      return "";

   // If a dot was found, the function continues.

   // Set $length to the length of the filename minus the position
   // of the dot.
   $length = strlen($filename) - $dot_position;

   // Set $ext to 
   $extension = substr($filename, $dot_position + 1, $length);

   // Make sure it's in lowercase
   $extension = strtolower($extension);

   return $extension;