Checking if a user has liked a specific page using Facebook PHP SDK 4.0.x and Graph API 2.0+

like_fb4.0.x.php

<?php

// get like status
$liked = (new FacebookRequest( $session, 'GET', '/me/likes/' . $page_id ))->execute()->getGraphObject()->asArray();

// if $liked return array with at least one result, user has liked the page
if ( isset( $liked ) && count( $liked ) > 0 ) {
  echo '<h2>Liked</h2>';
} else {
  echo '<h2>Not Liked</h2>';
}