//require sendgrid
require "sendgrid/sendgrid-php.php";

//sendMail function
function sendMail($to, $subject, $content, $from){

  //init sendgrid and email
  $sendgrid = new SendGrid("my_username", "my_password");
  $email = new SendGridEmail();

  //to address = individual or array
  if (is_array($to)) $email->setTos($to);
  else $email->addTo($to);

  //from, subject, contet
  $email->setFrom($from)->
          setSubject($subject)->
          setHtml($content);

  //send email
  $sendgrid->send($email);
}

sendMail("to@example.com", "Subject", "Content", "from@example.com");