$cmd = /usr/bin/dot -Tpng /home/users/name/public_html/pr/form.dot -o /home/users/name/public_html/pr/form.gif";
↓
$cmd = "/usr/bin/dot -Tpng /home/users/name/public_html/pr/form.dot -o /home/users/name/public_html/pr/form.gif";