GhostRider is pretty straight forward.  Example:

$ghost = new GhostRider("/usr/local/bin/"); // the location of 'gs' if your PHP does not execute it correctly by default. To find, enter 'which gs' in Command Line
$ghost->set_input("/var/www/example.com/html/public/temp/test.pdf");  // your input file
$ghost->add_option("-dSAFER");
$ghost->add_option("-dBATCH");
$ghost->add_option("-dNOPAUSE");
$ghost->add_option("-dGraphicsAlphaBits=4");
$ghost->add_option("-dTextAlphaBits=4");
$ghost->add_option("-sDEVICE=jpeg");
$ghost->add_option("-r300");
$ghost->add_option("-sOutputFile=/var/www/example.com/html/public/temp/test.jpg");
$ride = $ghost->ride(); // run the 'gs' command
if($ride)
  echo "Successful";
else
  echo "Error: " . $ride;