1°) As nobody replied here in the forum, and after few weeks looking for a solution, we decided to purchase some modules, instead of using the Odoo Demo account.

Then, I just changed the credentials for my new database, and also opened the 8069 port for that specific Address. And it worked :)

Code :

$url = 'https://thedatabasename.odoo.com';   // Edited here. And opening the firewall's 8069 port was the last step to make it work :)
$db = 'thedatabasename';                // Edited here
$username = 'usernameofthataccount';    // Edited here
$password = 'passwordofthataccount';    // Edited here

require_once('ripcord/ripcord.php');

$common = ripcord::client($url.'/xmlrpc/2/common');
$uid = $common->authenticate($db, $username, $password, array());

echo('UID:');
var_dump($uid);
echo('<br/>');

$models = ripcord::client("$url/xmlrpc/2/object");
$partners = $models->execute_kw(
    $db,
    $uid,
    $password,
    'res.partner',
    'search',
    array(
        array(
            array('is_company', '=', true)
        )
    )
);

echo('RESULT:<br/>');
foreach ($partners as $partner) {
    echo 'partner=['.$partner.']<br/>';
}
 
echo('VAR_DUMP:<br/>');
var_dump($partners);

Output :

UID:int(2)
RESULT:
partner=[568]
partner=[570]
partner=[293]
partner=[378]
partner=[526]
VAR_DUMP:
array(193) { [0]=> int(568) [1]=> int(570) [2]=> int(293) [3]=> int(378) [4]=> int(526)}

2°) The start( ) method didn't work with the specific Address though

So, I'm not sure that the start( ) method will work with something else than a Demo account.

Also, Odoo's Support told me to not include the "/start" part of the URL in the script. But, with or without the "/start" part, it didn't work.

Code :

require_once('ripcord/ripcord.php');
$info = ripcord::client('https://thedatabasename.odoo.com/start')->start();  // Even after using the specific Address (not the demo) and opened the 8069 port, this hasn't worked in my case

echo 'hello';

Output :

Fatal error: Uncaught Ripcord_TransportException: Could not access http://102.16.10.74:8069/start/ in /var/www/vhosts/mywebsiteexample.com/preprod.mywebsiteexample.com/ripcord/ripcord_client.php:488 Stack trace: #0 /var/www/vhosts/mywebsiteexample.com/preprod.mywebsiteexample.com/ripcord/ripcord_client.php(228): Ripcord_Transport_Stream->post('http://102.16.1...', '<?xml version="...') #1 /var/www/vhosts/mywebsiteexample.com/preprod.mywebsiteexample.com/index.php(11): Ripcord_Client->__call('start', Array) #2 {main} thrown in /var/www/vhosts/mywebsiteexample.com/preprod.mywebsiteexample.com/ripcord/ripcord_client.php on line 488

My Conclusion is :

Odoo DEMO has NOT worked for me, even if I followed the instructions from here :

https://www.odoo.com/documentation/14.0/webservices/odoo.html