cover_photo_upload.php

<?php

// include Facebook PHP SDK
include "facebook/facebook.php";

// init Facebook SDK with app settings
$facebook = new Facebook( array( 'appId' => APP_ID, 'secret' => APP_SECRET ) );

// enable file upload support
$facebook->setFileUploadSupport( true );

// set the page access token
$facebook->setAccessToken( PAGE_TOKEN );

// upload a photo to facebook, will return id of uploaded photo
$photo_uploaded = $facebook->api( $page_id . "/photos", "POST", array(
    'source' => '@' . 'cover.png',
    'no_story' => true // suppress automatic image upload story, optional
  ) );

// set uploaded photo as cover image, will return true on success
$cover = $facebook->api( $page_id, "POST", array(
    'cover' => $photo_uploaded['id'],
    'offset_x' => 0, // optional
    'offset_y' => 0, // optional
    'no_feed_story' => true // suppress automatic cover image story, optional
  ) );