Apologies - a bit later than anticipated, but hopefully this will help. I’ve used the “Invoice_Create” method as an example and added a few comments to help. This uses the JSON endpoints rather than the XML ones.
This hasn’t been tested and only created fairly quickly, so there may be errors. Hopefully it’ll point you in the right direction though 
<?php
$accountNumber = '6131400001'; // QuickFile account number
$applicationID = 'ABCD-100-EFGH'; // Application ID
$apiKey = 'HGFE-001-DCBA'; // Account API Key
$submissionCode = uniqid(); // Randomly generated submission number, but this can be anything unique
// End point
$endPoint = 'https://api.quickfile.co.uk/1_2/invoice/create';
// The data to be sent via the API
$data = [
'payload' => [
'Header' => [
'MessageType' => 'Request',
'SubmissionNumber' => $submissionCode,
'Authentication' => [
'AccNumber' => $accountNumber,
'MD5Value' => md5($accountNumber . $apiKey . $submissionCode),
'ApplicationID' => $applicationID
]
],
'Body' => [
'InvoiceData' => [
'InvoiceType' => 'INVOICE',
'ClientID' => 12345,
'ClientAddress' => [
'Address' => '123 High Street<br>London<br>AA1 1AA<br>United Kingdom',
'CountryISO' => 'GB'
],
'Currency' => 'GBP',
'TermDays' => 14,
'Language' => 'en',
'InvoiceLines' => [
'ItemLines' => [
'ItemLine' => [
'ItemID' => 0,
'ItemName' => 'USB Cable',
'ItemDescription' => '2.5m USD Cable (Blue)',
'ItemNominalCode' => 4000,
'Tax1' => [
'TaxName' => 'VAT',
'TaxPercentage' => '20.00',
'TaxAmount' => '2'
],
'UnitCost' => '10.00',
'Qty' => 1
]
]
],
'Scheduling' => [
'SingleInvoiceData' => [
'IssueDate' => '2018-01-25'
]
]
]
]
]
];
// Set up the cURL handler
$ch = curl_init($endPoint);
// Set up options, including data
curl_setopt_array($ch, array(
CURLOPT_POST => TRUE,
CURLOPT_RETURNTRANSFER => TRUE,
CURLOPT_HTTPHEADER => array(
'Content-Type: text/json'
),
CURLINFO_HEADER_OUT => TRUE,
CURLOPT_POSTFIELDS => json_encode($data),
CURLOPT_SSL_VERIFYPEER => TRUE // This may need to be false if testing from localhost
));
// Send the request
$response = curl_exec($ch);
// Get HTTP Code
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
// Decode the returned JSON
$return = json_decode($response);
// Check for errors
if($httpCode != 200)
{
// Something didn't go right
// $return->Errors contains the errors returned
}
else
{
// All good!
// $return will contain the returned data inline with the API Docs
}
// Note: Not tested, but hopefully it'll point you in the right direction if nothing else
Full documentation for this function can be found here:
https://api.quickfile.co.uk/d/v1_2/Invoice_Create