$pdf_file  = './pdf/demo.pdf';
$save_to  = './jpg/demo.jpg';
//make sure that apache has permissions to write in this folder! 
//(common problem)
 
//execute ImageMagick command 'convert' and convert PDF
//to JPG with applied settings
exec('convert "'.$pdf_file.'" -colorspace RGB -resize 800 "'.$save_to.'"', $output, $return_var);
 
if($return_var == 0) {
//if exec successfuly converted pdf to jpg
  print "Conversion OK";
}
else print "Conversion failed.".$output;