Detect JPEG markers in string - http://en.wikipedia.org/wiki/JPEG

isJpeg.function.php

function isJpeg($string) {
  if(ord($string[0]) != 0xFF) {
    return false;
  }

  if(ord($string[1]) != 0xD8) {
    return false;
  }

  $count = strlen($string);

  if(ord($string[$count-2]) != 0xFF) {
    return false;
  }

  if(ord($string[$count-1]) != 0xD9) {
    return false;
  }

  return true;
}