ListOAuthCredentials

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://www.example.com/includes/api.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,
    http_build_query(
        array(
            'action' => 'ListOAuthCredentials',
            // See https://developers.whmcs.com/api/authentication
            'username' => 'IDENTIFIER_OR_ADMIN_USERNAME',
            'password' => 'SECRET_OR_HASHED_PASSWORD',
            'responsetype' => 'json',
        )
    )
);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec($ch);
curl_close($ch);

$command = 'ListOAuthCredentials';
$postData = array(
);
$adminUsername = 'ADMIN_USERNAME'; // Optional for WHMCS 7.2 and later

$results = localAPI($command, $postData, $adminUsername);
print_r($results);

{
    "result": "success",
    "clients": [
        {
            "credentialId": 1,
            "name": "",
            "description": "",
            "grantTypes": "single_sign_on",
            "scope": "clientarea:sso clientarea:profile clientarea:billing_info clientarea:emails clientarea:announcements clientarea:downloads clientarea:knowledgebase clientarea:network_status clientarea:product_details clientarea:invoices clientarea:tickets clientarea:submit_ticket clientarea:shopping_cart_addons clientarea:shopping_cart_domain_register clientarea:shopping_cart_domain_transfer",
            "clientIdentifier": "COMPANY-NAME.+ddPpB4+MVU5jeacL\/iEiw==",
            "clientSecret": "cl\/1zwMst0mtoAFLrTL\/3PAneaOXJ51ipoY86sKkapZM3WPKEwUK\/jhxXOAn2ErqXZNTJFnv2isNyYE0fgjgqQ==",
            "uuid": null,
            "serviceId": 1,
            "logoUri": "",
            "redirectUri": [],
            "rsaKeyPairId": 0,
            "createdAt": "18th November 2016 9:32:24am",
            "updatedAt": "18th November 2016 9:32:24am"
        },
        {
            "credentialId": 2,
            "name": "",
            "description": "",
            "grantTypes": "single_sign_on",
            "scope": "clientarea:sso clientarea:profile clientarea:billing_info clientarea:emails clientarea:announcements clientarea:downloads clientarea:knowledgebase clientarea:network_status clientarea:product_details clientarea:invoices clientarea:tickets clientarea:submit_ticket clientarea:shopping_cart_addons clientarea:shopping_cart_domain_register clientarea:shopping_cart_domain_transfer",
            "clientIdentifier": "COMPANY-NAME.98x4Dkq4JXjj3SpAqn14+Q==",
            "clientSecret": "uKw1nmFF9XOHUynCHIcAGBmFsrlMHrD\/5GyUyY\/Ghw4toDI8wQ63XqY6maAtF5Vb02SgI6tqyNbd1BmPzPd9AQ==",
            "uuid": null,
            "serviceId": 1,
            "logoUri": "",
            "redirectUri": [],
            "rsaKeyPairId": 0,
            "createdAt": "18th November 2016 9:33:01am",
            "updatedAt": "18th November 2016 9:33:01am"
        },
        {
            "credentialId": 3,
            "name": "",
            "description": "",
            "grantTypes": "single_sign_on",
            "scope": "clientarea:sso clientarea:profile clientarea:billing_info clientarea:emails clientarea:announcements clientarea:downloads clientarea:knowledgebase clientarea:network_status clientarea:product_details clientarea:invoices clientarea:tickets clientarea:submit_ticket clientarea:shopping_cart_addons clientarea:shopping_cart_domain_register clientarea:shopping_cart_domain_transfer",
            "clientIdentifier": "COMPANY-NAME.mY7l7Iz9NuXOeUleDM8rpQ==",
            "clientSecret": "O12n9lVX5OMv2SgMJ3u+p7q\/RVhuXybwdNbNOnDBdAci4U3kD3X8H1M9WRr4NuUb9fPTMbD3ySxUan9qf8gsdQ==",
            "uuid": null,
            "serviceId": 1,
            "logoUri": "",
            "redirectUri": [],
            "rsaKeyPairId": 0,
            "createdAt": "18th November 2016 9:35:20am",
            "updatedAt": "18th November 2016 9:35:20am"
        }
    ]
}