smtpapiheaders.php

// This is the example from https://github.com/sendgrid/sendgrid-php#headers
// but slight modifications have been made to include code for IP Pooling

$email = new SendGrid\Email();
$email
    ->addTo('foo@bar.com')
    ->addTo('bar@foo.com')
    ->setFrom('me@bar.com')
    ->setSubject('Subject goes here')
    ->setText('Hello World!')
    ->setHtml('<strong>Hello World!</strong>')
    ->addHeader('X-Smtpapi', {'ip_pool': 'pool_name'})
;

// Send it
$sendgrid->send($email);