### main function:

function disp($pid) {
      if (file_exists("pic-".$pid.".JPG")) {
            echo "<img src=pic-".$pid.".JPG>";
            echo "<br><br>";
            if (file_exists("pic-".($pid-1).".JPG")) {
                  echo "Previous &nbsp; ";
            }
            if (file_exists("pic-".($pid+1).".JPG")) {
                  echo "Next";
            }
      } else {
            echo "this file doesn't exist, try here";
      }
}

### code for displaying:

if ($_GET['pid']) {
      disp($_GET['pid']);
} else {
      disp(1);
}