Erreur PHP application facebook

Bonjour,

Je suis actuellement en train d'essayer de développer une application facebook en PHP en utilisant la librairies PHP facebook.

Je n'ai pour l'instant rien fait et cela bug déjà.

J'ai ce message d'erreur: Catchable fatal error: Object of class Facebook could not be converted to string in /home/exo979/www/poemes/footprints/lib.php on line 4

Mon code lib.php est celui-ci:

<?php

function get_db_conn() {
$conn = mysql_connect($GLOBALS['localhost'], $GLOBALS['nom de ma BDD'], $GLOBALS['mdp de ma BDD']);
mysql_select_db($GLOBALS['app_facebook'], $conn);
return $conn;
}

function get_prints($user) {
$conn = get_db_conn();
$res = mysql_query('SELECT `from`, `to`, `time` FROM footprints WHERE `to`=' . $user . ' ORDER BY `time` DESC', $conn);
$prints = array();
while ($row = mysql_fetch_assoc($res)) {
$prints[] = $row;
}
return $prints;
}

function render_profile_action($id, $num) {
return '<fb:profile-action url="https://apps.facebook.com/login/?next=https%3A%2F%2Fapps.facebook.com%2Ffootprints%2F%3Fto%3D%2527 . $id . '">'
. '<fb:name uid="' . $id . '" firstnameonly="true" capitalize="true"/> '
. 'has been stepped on ' . $num . ' times.'
. '</fb:profile-action>';
}

function render_profile_box($id, $prints) {
// Render the most recent 5 no matter what, and the second most recent 5
// only if the box is on the right (wide) side of the profile.
return render_prints($prints, 5) . '<fb:wide>' . render_prints(array_slice($prints, 5), 5) . '</fb:wide>'
. '<div style="clear: both;">' . render_step_link($to) . '</div>';
}

function do_step($from, $to) {
global $facebook;

$conn = get_db_conn();
mysql_query('INSERT INTO footprints SET `from`='.$from.', `time`='.time().', `to`='.$to, $conn);
$prints = get_prints($to);
try {

// Set Profile FBML
$fbml = render_profile_action($to, count($prints)) . render_profile_box($to, $prints);

// start batch operation
$facebook->api_client->begin_batch();

$facebook->api_client->profile_setFBML($fbml, $to);

// Send notification
// Notice the use of reference '&'
$result = & $facebook->api_client->notifications_send($to, ' stepped on you. ' .
'<a href="https://apps.facebook.com/login/?next=https%3A%2F%2Fapps.facebook.com%2Ffootprints%2F">See all your Footprints</a>.');

// Publish feed story
$feed_title = '<fb:userlink uid="'.$from.'" shownetwork="false"/> stepped on <fb:name uid="'.$to.'"/>.';
$feed_body = 'Check out <a href="https://apps.facebook.com/login/?next=https%3A%2F%2Fapps.facebook.com%2Ffootprints%2F%3Fto%3D%2527$to.'">' .
'<fb:name uid="'.$to.'" firstnameonly="true" possessive="true"/> Footprints</a>.';
$facebook->api_client->feed_publishActionOfUser($feed_title, $feed_body);

// End batch operation. This will actually send queued API call to Facebook in
// a single HTTP request
$facebook->api_client->end_batch();

} catch (Exception $e) {
error_log($e->getMessage());
}
return $prints;
}

function render_step_link($id) {
return '<a href="https://apps.facebook.com/login/?next=https%3A%2F%2Fapps.facebook.com%2Ffootprints%2F%3Fto%3D%2527 . $id . '">'
. 'Step on <fb:name uid="' . $id . '" firstnameonly="true"/>'
. '</a>';
}

function render_prints($prints, $max) {
$fbml = '';
$i = 0;
foreach ($prints as $post) {
$fbml .= '<fb:if-can-see uid="' . $post['from'] . '"><div style="clear: both; padding: 3px;">'
. '<fb:profile-pic style="float: left;" uid="' . $post['from'] . '" size="square"/>'
. '<fb:name uid="' . $post['from'] . '" capitalize="true"/> stepped on <fb:name uid="' . $post['to'] . '"/>'
. ' at <fb:time t="' . $post['time'] . '"/>. '
. '<br/>' . render_step_link($post['from']) . '<br/>'
. '</div></fb:if-can-see>';
if (++$i == $max) break;
}
return $fbml;
}

Que veux dire exactement cette phrase d'erreur et comment puis-je la corriger ?