Each Infusionsoft account has an API key that allows you to make calls to the API.
Here are the directions to get the API key for your Infusionsoft application.
http://ug.infusionsoft.com/article/AA-00442/0/Infusionsoft-API-Key.html
Once you have the key you can use the PHP SKD to make calls and add contacts. Here is a link to the infusionosft php SDK:
https://github.com/infusionsoft/infusionsoft-php
Here is a link to the documentation to add a contact along with a php example:
https://developer.infusionsoft.com/docs/xml-rpc/#contact
https://github.com/infusionsoft/API-Sample-Code/blob/master/PHP/ContactService-Sample.php
Edit
It looks like they are eventually sunsetting account-level keys in the future which did not require you use to oauth.
https://developer.infusionsoft.com/2014/07/03/simplifying-infusionsoft-authentication-with-oauth2/
There are plenty of examples on how to use oauth with Infusionsoft here:
https://developer.infusionsoft.com/docs/xml-rpc/#contact
Click PHP on the right hand side and you will see how to get a token and create a contact with the API
There are even more examples on the README in github:
https://github.com/infusionsoft/infusionsoft-php/blob/master/README.md
Authentication:
$infusionsoft = new \Infusionsoft\Infusionsoft(array(
'clientId' => 'XXXXXXXXXXXXXXXXXXXXXXXX',
'clientSecret' => 'XXXXXXXXXX',
'redirectUri' => 'http://example.com/',
));
// If the serialized token is available in the session storage, we tell the SDK
// to use that token for subsequent requests.
if (isset($_SESSION['token'])) {
$infusionsoft->setToken(unserialize($_SESSION['token']));
}
// If we are returning from Infusionsoft we need to exchange the code for an
// access token.
if (isset($_GET['code']) and !$infusionsoft->getToken()) {
$infusionsoft->requestAccessToken($_GET['code']);
}
if ($infusionsoft->getToken()) {
// Save the serialized token to the current session for subsequent requests
$_SESSION['token'] = serialize($infusionsoft->getToken());
// MAKE INFUSIONSOFT REQUEST
} else {
echo '<a href="' . $infusionsoft->getAuthorizationUrl() . '">Click here to authorize</a>';
}