how to decrypt password in this script?

 * generates a random string
 *
 * @param int $length
 * @param string $characters
 * @return string
 */
function random_string($length=8,$characters='abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789')
 {
  $random_string = '';
  $characters_length = strlen($characters);
  for($i=0;$i<$length;$i++)
   {
    $random_string .= $characters[mt_rand(0, $characters_length - 1)];
   }
  return $random_string;
 }
 
/**
 * generates password hash
 *
 * @param string $pw
 * @return string
 */
function generate_pw_hash($pw)
 {
  $salt = random_string(10,'0123456789abcdef');
  $salted_hash = sha1($pw.$salt);
  $hash_with_salt = $salted_hash.$salt;
  return $hash_with_salt;
 }
 
/**
 * checks password comparing it with the hash
 *
 * @param string $pw
 * @param string $hash
 * @return bool
 */
function is_pw_correct($pw,$hash)
 {
  if(strlen($hash)==50) // salted sha1 hash with salt
   {
    $salted_hash = substr($hash,0,40);
    $salt = substr($hash,40,10);
    if(sha1($pw.$salt)==$salted_hash) return true;
    else return false;
   }
  elseif(strlen($hash)==32) // md5 hash generated in an older version
   {
    if($hash == md5($pw)) return true;
    else return false;
   }
  else return false;
 }