gistfile1.txt

$thumbUrl = $item->get('thumbnail',array(
    'w' => (int)$modx->getOption('imageWidth',$scriptProperties,500),
    'h' => (int)$modx->getOption('imageHeight',$scriptProperties,500),
    'zc' => (boolean)$modx->getOption('imageZoomCrop',$scriptProperties,0),
    'far' => (string)$modx->getOption('imageFar',$scriptProperties,false),
    'q' => (int)$modx->getOption('imageQuality',$scriptProperties,90),
));

/* or */
$thumbUrl = $item->get('thumbnail',array(
 'w' => 300,
 'h' => 200,
 'q' => 100,
 'otherPhpThumbParam' => 'value',
));