if (is_file('some_dir/some_file.php'))
	include_once ('some_dir/some_file.php');

  if (is_file($_SERVER['DOCUMENT_ROOT'] . '/myfile.jpg'))
    echo "FILE EXISTS";

Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.