Salesforce PHP SOAP API。 如何填充查阅字段？

I have a custom object called "Chapter_Detail__c". I am creating a LEAD, and a lead has a look up field(link_id__c) which is mapped with "Chapter_Detail__c".

How can i populate this look up field while creating the Lead.

$sObject = new SObject();
$fieldsToInsert = array (
  'FirstName' => $name_rsform,
  'LastName' => $username_rsform,
  'Company' => 'TestSession',
  'Full_Name__c' => $name_rsform,
  'Chapter_Name__c' => $chapter_name_rsform,
  'Role__c' => $role_rsform_label,
  'Email__c' => $email_rsform,
  'Branch_of_service__c' => $bos_f,

    // 'link_id__c' => $chapter_id_rsform, - DOES NOT WORK THIS WAY ITS A LOOKUP RELATIONSHIP WITH OBJECT "Chapter_Detail__c"
  );
 $sObject->fields = $fieldsToInsert;
 $sObject->type = 'Lead';
    //echo "**** Creating the following:
";
 $createResponse = $mySforceConnection->create(array($sObject));
    //print_r($createResponse);

 echo 'Get ID after create';
$ids = array();
foreach ($createResponse as $createResult) {
    print_r($createResult);
    array_push($ids, $createResult->id);
}

$idRecieved = $ids[0];

Any kind of help will be appreciated. Thanks.