Facebook PHP SDK - check access token validity

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<?php

// this follows the assumption that you've already instantiated the facebook SDK
// e.g. $facebook = new Facebook();

// The key is the first api() call - you need to try to access something which requires the access token
// you are attempting to validate.  
// e.g. if your access token has *no* special permissions you could use $facebook->api('/me'),
// which is the most basic permission level.
try {
	$facebook->api('/me');
} catch (Exception $e) {
	if ($e->getType() == "OAuthException") {
		$url = $this->facebook->getLoginUrl(array(
	        'redirect_uri' => "<your redirect uri which has to be set as owned by the app>"
	    ));
   		echo "<script type='text/javascript'>top.location.href = '$url';</script>";
	}
}