I have really struggled with the Linnworks API documentation, no examples and nothing on Google to reference... so I thought I would share a little work to assist anyone else struggling;

(1) If you haven't already you'll need to register your API, obtaining your own unique applicationId, applicationSecret & token (you'll need these, they shouldn't be shared); http://apps.linnworks.net/Applications

(2) You'll need the Linnworks SDK, go ahead and download it from here; https://github.com/LinnSystems/LinnworksNetSDK.git

(3) House the files where you can reference from your web root, I put mine below root in an Includes folder and include like this in your php file (They are more files names according to the call you are making).

require_once('../inc/Linnworks/src/php/Factory.php');
require_once('../inc/Linnworks/src/php/Stock.php');

   (4) Next Include the authorisation you obtained in step one;

$authorization = json_decode(Factory::GetResponse("Auth/AuthorizeByApplication", "applicationId=HERE&applicationSecret=HERE&token=HERE", "", "https://api.linnworks.net/"));

 (5) Next make your first call, here we are doing a SKU search for the SKU "TOYCAR";

$StockItemsFull = json_decode(Factory::GetResponse("Stock/GetStockItemsFull","keyword=TOYCAR&loadCompositeParents=true&loadVariationParents=true&entriesPerPage=1&pageNumber=1&dataRequirements=[1,2]&searchTypes=[SKU]",$authorization->Token,"https://api.linnworks.net/"));

 (6) Your results will be returned as an array;

echo "<pre>"; print_r($StockItemsFull); echo "</pre>";

 Hope this helps you get started 

<?php require_once('../inc/Linnworks/src/php/Factory.php');
require_once('../inc/Linnworks/src/php/Stock.php');
$authorization = json_decode(Factory::GetResponse("Auth/AuthorizeByApplication", "applicationId=HERE&applicationSecret=HERE&token=HERE", "", "https://api.linnworks.net/"));
$StockItemsFull = json_decode(Factory::GetResponse("Stock/GetStockItemsFull","keyword=TOYCAR&loadCompositeParents=true&loadVariationParents=true&entriesPerPage=1&pageNumber=1&dataRequirements=[1,2]&searchTypes=[SKU]",$authorization->Token,"https://api.linnworks.net/"));
echo "<pre>"; print_r($StockItemsFull); echo "</pre>"; ?>

Just wanted to mention that searchTypes=[SKU] seems to be invalid now, I get:

{
    "Code": "-",
    "Message": "Can't deserialize \"searchTypes\" parameter. Error: \"Unexpected character encountered while parsing value: S. Path '', line 1, position 1.\""
}

http://apps.linnworks.net/Api/Class/linnworks-spa-commondata-Inventory-ClassBase-Enums-StockInformationSearchType indicates that 'SKU' is the first valid enum, you just have to provide 0 instead of SKU, i.e.:

keyword=TOYCAR&loadCompositeParents=true&loadVariationParents=true&entriesPerPage=1&pageNumber=1&dataRequirements=[1,2]&searchTypes=[0]

The Factory just returns null (because Factory::GetResponse uses file_get_contents and doesn't actually present the error response to you, I had to debug it via Postman to see the error message) so I hope this helps someone.

By the way, in your second call you should use the Server value returned in $authorization.  http://apps.linnworks.net/Api says "Servers

When you have authorized an account, you will be given a session token and a session server. You must use these details to access the account from then onwards."  So your second call becomes:

$StockItemsFull = json_decode(Factory::GetResponse("Stock/GetStockItemsFull","keyword=TOYCAR&loadCompositeParents=true&loadVariationParents=true&entriesPerPage=1&pageNumber=1&dataRequirements=[1,2]&searchTypes=[SKU]",$authorization->Token, $authorization->Server));

There is code in Factory::GetResponse that adds the "/api/" part of the URL for you.