<?PHP
 
FUNCTION strhex($string)
 {
    $hex = "";
    $len = STRLEN($string);
 
    FOR($i = 0; $i < $len; ++$i)
     {
       $hex .= STR_PAD(DECHEX(ORD($string[$i])), 2, 0, STR_PAD_LEFT);
     }
    RETURN $hex;
 }
 
$data = FILE_GET_CONTENTS($filename);
 
IF(PREG_MATCH('/\.(jpg|jpeg|gif|png)/i', $filename, $matches))
 {
    $type = ($matches[1] === 'jpeg') ? 'jpg' : $matches[1];
 }
ELSE
 {
     // Get the first 12 bytes of the file
     $hdr  = strhex($data[0].$data[1].$data[2].$data[3].$data[4].$data[5].$data[6].$data[7].$data[8].$data[9].$data[10].$data[11]);
     ECHO $hdr;
 
     // Match header of the file (Google it).
     IF(PREG_MATCH('$/ffd8ffe0....4A46494600/i', $hdr))
      {
        // JPEG?
        $type = 'jpg';
      }
     ELSEIF(PREG_MATCH('$/89504e470D0a1a0a/i', $hdr))
      {
        // PNG?
        $type = 'png';
      }
     ELSEIF(PREG_MATCH('$/474946/i', $hdr))
      {
        // GIF?
        $type = 'gif';
      }
     ELSE
      {
         // Unknown image type.
      }
 }