like_gate.php

<?php

/********************************************************
 * Usage: 
 *  - include this file in any document
 *  - use the following PHP tags to show / hide content:
 *
 *  <?php if( LIKED ): ?> 
 *      Welcome my friend
 *  <?php else: ?> 
 *      Please like us
 *  <?php endif; ?> 
 *
 *******************************************************/

$signed_request = $_REQUEST['signed_request'];

function parsePageSignedRequest() 
{
    if ( isset( $_REQUEST['signed_request'] ) ) 
    {
		$encoded_sig = null;
		$payload = null;
		list($encoded_sig, $payload) = explode('.', $_REQUEST['signed_request'], 2);
		$sig = base64_decode(strtr($encoded_sig, '-_', '+/'));
		$data = json_decode(base64_decode(strtr($payload, '-_', '+/'), true));
		return $data;
    }
    return false;
}

if( $signed_request = parsePageSignedRequest() ) 
{
    if( $signed_request->page->liked ) 
    {
		define( 'LIKED', TRUE );
    } 
    else 
    {
		define( 'LIKED', FALSE );
    }
}