$acctRole=array();
 $acctRole[0] = new stdclass();
 $acctRole[0]->AccountId=$this->accountId;
 $acctRole[0]->ContactId=$this->contactId;
 $acctRole[0]->IsPrimary=1;
 $acctRole[0]->Role="Buyer";
 $response = $this->mySforceConnection->create($acctRole, 'AccountContactRole');