create_fb_event.php

<?php

require __DIR__.'/vendor/facebook/src/facebook.php';

$app_id = '1234567890';
$secret = '0123456789abcdef0123456789abcdef0123456789abcdef';

$fb = new Facebook(array(
    'appId'      => $app_id,
    'secret'     => $secret,
    'cookie'     => false,
    'fileUpload' => true     // this is important !
));

// alternatively you can enable file upload on the instance
// $fb->setFileUploadSupport(true);

// the image you want to use on your event
$file = realpath('/path/to/your/example.jpg');

$eventInfo = array(
    'name'          => 'some title',
    'start_time'    => time()+32400,
    'description'   => 'some description',
    basename($file) => '@'.$file,
    // `-> 'example.jpg' => '@/path/to/your/example.jpg'
);

$result = $fb->api(
    '/'.$app_id.'/events',
    'post',
    $eventInfo
);

echo 'Created event #'.$result['id'];