From Magento 2 admin panel, create a new user with a new role with sales, customers and catalog permisions. Change baseURL, username and password lines 9-11. Change example order id, customer id and product sku if needed (lines 31, 38, 45). Run from cli: php magento_api.php

magento_api.php

<?php
class Runner {
    public $baseURL;
    public $username;
    public $password;
    public $token_string;

    function __construct() {
        $this->baseURL = "https://domain.com/index.php";
        $this->username = "magento_api";
        $this->password = "whatever";
        $this->token_string = null;
    }

    function run(){
        $this->auth();
        $this->print_example_order();
        $this->print_example_customer();
        $this->print_example_product();
    }
    function auth(){
        $data = array("username" => $this->username, "password" => $this->password);
        $response = $this->request("/rest/V1/integration/admin/token", $data);
        $this->token_string = json_decode($response);
        echo "\nauth()\n";
        echo "token:".$this->token_string;
        echo "\n";
    }
    function print_example_order(){
        echo "\nprint_example_order()\n";
        $response = $this->request("/rest/V1/orders/29229");
        $order = json_decode($response);
        print_r($order);
        echo "\n";
    }
    function print_example_customer(){
        echo "\nprint_example_customer()\n";
        $response = $this->request("/rest/V1/customers/5389");
        $customer = json_decode($response);
        print_r($customer);
        echo "\n";
    }
    function print_example_product(){
        echo "\nprint_example_product()\n";
        $response = $this->request("/rest/V1/products/PEZ0140");
        $category = json_decode($response);
        print_r($category);
        echo "\n";
    }
    function request($requestUrl,  $data = null){
        $ch = curl_init($this->baseURL.$requestUrl);
        if(!$this->token_string && $data != null){
            $data_string = json_encode($data);
            curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
            curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);
            curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
            curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/json","Content-Length: ".strlen($data_string)));

        }
        if($this->token_string && $data == null){
            $headers = array("Authorization: Bearer ".$this->token_string);
            curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        }
        return curl_exec($ch);
    }
}
$magento_api = new Runner();
$magento_api->run();

?>