Tableau REST Api example (in PHP) - List workbooks by tags

<?php 

$username = "_USERNAME_";
$password = "_PASSWORD_";
$sitename = ""; // blank for default
$URL = "_URL_";

////////////////
// Find Token (/api/2.0/auth/signin)
////////////////

$token = curlTableauToken($username, $password, $URL);


////////////////
// Query Site ID (/api/2.0/sites)
////////////////

$xml = curlTableauXML($URL."/api/2.0/sites", $token);
foreach($xml->sites->site as $site){
	if($site->attributes()->contentUrl == $sitename){
		$siteID = $site->attributes()->id;
	}
}


////////////////
// Query User ID
/////////////////

$xml = curlTableauXML($URL."/api/2.0/sites/$siteID/users", $token);
foreach($xml->users->user as $user){
	if($user->attributes()->name == $username){
		$userID = $user->attributes()->id;
	}
}


////////////////
// Query Workbooks on the site, from the user
/////////////////

$xml = curlTableauXML($URL."/api/2.0/sites/$siteID/users/$userID/workbooks", $token);
foreach($xml->workbooks->workbook as $workbook){
	$id = $workbook->attributes()->id->__toString();
	$books[$id]["name"] = $workbook->attributes()->name;
	$books[$id]["contentUrl"] = $workbook->attributes()->contentUrl;

	foreach($workbook->tags->tag as $tag){
		$name = $tag->attributes()->label->__toString();
		$tags[$name][] = $id;
	}	
}

////////////////
// Query First view in every workbook on the site, from the user
/////////////////

foreach($books as $id=>$book){
	$xml = curlTableauXML($URL."/api/2.0/sites/$siteID/workbooks/$id/views", $token);
	$books[$id]["view"] = $xml->views->view[0]->attributes()->contentUrl;
}


////////////////
// Layout
/////////////////
foreach($tags as $tag=>$wb){
	echo "<h2>".$tag."</h2>";
	
	foreach($wb as $id){
		$url = "$URL/".(($sitename == "") ? "" : "site/".$sitename)."views/".str_replace("sheets/","",$books[$id]["view"]);
		echo "<h4><a href='$url'>".$books[$id]["name"]."</a></h4>";
	}
}
 
 
////////////////
// Functions
/////////////////
function curlTableauXML($url, $token = ""){

	$ch = curl_init($url);
	curl_setopt($ch, CURLOPT_HTTPHEADER, array("X-tableau-auth: $token"));
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
	$output = curl_exec($ch);
	curl_close($ch);
	$xml = simplexml_load_string($output);
	
	if($xml->error->detail != null){
		die("Error: ".$xml->error->detail);
	}
	
	return $xml;
	
}

function curlTableauToken($username,$password,$URL){

	$payload = '
	<tsRequest>
	<credentials name="'.$username.'" password="'.$password.'" >
	<site contentUrl="" />
	</credentials>
	</tsRequest>
	';
	
	$ch = curl_init($URL."/api/2.0/auth/signin");
	curl_setopt($ch, CURLOPT_POST, 1);
	curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: text/xml'));
	curl_setopt($ch, CURLOPT_POSTFIELDS, $payload);
	curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
	$output = curl_exec($ch);
	curl_close($ch);

	$xml = simplexml_load_string($output);
	$token = $xml->credentials->attributes()->token;
	return $token;
	
}
 

Please tell how to use it for server with no sites?