So, again, this integration task consists of two parts: first, the user must search for a friend whom they wish to invite to your site and then the invitation must be sent.

1. The search function is outlined very well in the LinkedIn API documentation so we’ll skip here directly to a simple example in order to highlight the most important features:

//searching of all friends named Ann
 
$search_response = $linkedin->search("?facets=network&facet=network,F&first-name=Ann");
 
$xml = simplexml_load_string($search_response);
 
foreach($xml->people->person as $person)
{
print_r($person);
}

Take special note:

• All parameters of a search inquiry are connected with logical AND.

2. Unfortunately LinkedIn does not allow us to detect the email address of a user but invitations can be sent via LinkedIn’s on-site messaging system. While the API will allow us to send on-site messages, the feature is not implemented in the linkedin.php library which we’ve loaded and so we’ll need to modify a bit. Add this code into the file linkedin.php:

• WildCard is not supported.
• If a search returns too many results you are able to narrow them done using the “total” parameter in the returned XML:

base_url . "/v1/people/~/mailbox";
$request = OAuthRequest::from_consumer_and_token($this->consumer, $this->access_token, "POST", $send_url);
$request->sign_request($this->signature_method, $this->consumer, $this->access_token);
$auth_header = $request->to_header("https://api.linkedin.com");
 
if ($this->debug) {
echo $request->get_signature_base_string() . "\n";
echo $auth_header . "\n";
}
 
$raw_xml = "
 
  $subj
  $msg
";
$status = 0;
$response = $this->httpRequest($send_url, $auth_header, "POST", $raw_xml, $status);
 
return $status;
}
?>

Now the code to send the message will look like this:

//$id_rec - id recipient in LinkedIn.
$send_response = $linkedin->sendMessage($id_rec, 'Subject', 'Message body');
 
if($send_response == 201)
{
//Message sended
}
else
{
//Error handling
}

Talking about “integrating” is one thing; how does one integrate in practice? LinkedIn provides its members with a simple, well-documented API to allow for this and other off-site functions. The OAuth protocol, used by LinkedIn to authorize its account holders, allows you to integrate the content you create on LinkedIn with your website without the need to store the account password locally. While working with OAuth is not necessarily a simple thing, free libraries like this one (that I personally use) exist on the web to make the process an easier one.

The first step is to register your site with LinkedIn; this will in turn provide you with the API key and Secret key, each of which is needed to proceed. The keys are used to authorize users in these steps:

1. First we’ll create a temporary token for the user; this will later be used until a permanent access token is generated:

$linkedin = new LinkedIn($linkedin_api_key, $linkedin_secret_key, $linkedin_callback_url);
 
$linkedin->getRequestToken();

Here, $linkedin_api_key and $linkedin_secret_key are the keys obtained from LinkedIn upon registering of our application, and $linkedin_callback_url is the url on your site where the user will be redirected after authorizing.

Now a Request token is generated and gets in $linkedin->request_token where it must be retained, for example in a database.

2. Direct the user to a custom URL, formatted with the help of a Request token

header("Location: " . $linkedin->generateAuthorizeUrl());

Here the user will see a form for authorization where they’ll need to enter their email address and password. Following this, the user will be redirected back to your site, specifically to an URL which will be defined when the object is created by LinkedIn.

3. Now we need to create an object called LinkedIn and include within it all of the data necessary to generate an Access Token.

$linkedin = new LinkedIn($linkedin_api_key, $linkedin_secret_key, $linkedin_callback_url);
 
$linkedin->request_token = '';
 
$linkedin->oauth_verifier = $_REQUEST['oauth_verifier'];

Here, $ _REQUEST ['oauth_verifier'] is a variable passed by LinkedIn.

4. Now we can generate the Access Token.

$linkedin->getAccessToken($oauth_verifier);

The code: $ linkedin-> access_token must be stored in the database for future usage.

All of these actions must be completed only during the first connection of a user to LinkedIn from your website; in the future, the same user will be able to connect to LinkedIn using the Access Token in your database:

$linkedin = new LinkedIn($linkedin_api_key, $linkedin_secret_key, $linkedin_callback_url);
 
$linkedin->access_token = < obtained from the database >;
 
// obtain data about the user
 
$xml_response = $linkedin->getProfile("~:(id,first-name,last-name,headline,picture-url)");