Exchange authorization code

get_access_token.php

// Set variables for our request
$query = array(
  "client_id" => $api_key, // Your API key
  "client_secret" => $shared_secret, // Your app credentials (secret key)
  "code" => $params['code'] // Grab the access key from the URL
);

// Generate access token URL
$access_token_url = "https://" . $params['shop'] . "/admin/oauth/access_token";

// Configure curl client and execute request
$ch = curl_init();
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_URL, $access_token_url);
curl_setopt($ch, CURLOPT_POST, count($query));
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($query));
$result = curl_exec($ch);
curl_close($ch);

// Store the access token
$result = json_decode($result, true);
$access_token = $result['access_token'];

From https://github.com/nyalex/shopify-generating-api-token-guide/blob/master/generate_token.php