Product Link Types

$client = new SoapClient('http://magentohost/api/soap/?wsdl');

// If somestuff requires API authentication,
// then get a session token
$session = $client->login('apiUser', 'apiKey');

$result = $client->call($session, 'product_link.types');
var_dump($result);

// If you don't need the session anymore
//$client->endSession($session);

$proxy = new SoapClient('http://magentohost/api/v2_soap/?wsdl'); // TODO : change url
$sessionId = $proxy->login('apiUser', 'apiKey'); // TODO : change login and pwd if necessary

$result = $proxy->catalogProductLinkTypes($sessionId);
var_dump($result);

$proxy = new SoapClient('http://magentohost/api/v2_soap/?wsdl');
$sessionId = $proxy->login((object)array('username' => 'apiUser', 'apiKey' => 'apiKey'));

$result = $proxy->catalogProductLinkTypes((object)array('sessionId' => $sessionId->result));

var_dump($result->result);

array
  0 => string 'related' (length=7)
  1 => string 'up_sell' (length=7)
  2 => string 'cross_sell' (length=10)
  3 => string 'grouped' (length=7)