Consider the following class
<?php class Customer { private $id; private $firstname; private $surname; private $email; public function __construct( int $id, string $firstname, string $surname, string $email ) { $this->id = $id; $this->firstname = $firstname; $this->surname = $surname; $this->email = $email; } } ?>
Suppose we save it to current folder with file name Customer.php.
To use it in another file, use the require command.
require_once __DIR__ . '/Customer.php'; $customer1 = new Customer(1, 'John', 'Doe', 'a@mail.com');
You do not need to include the files every single time.
Once you include them, PHP will know where to find the classes, even though your code is in a different file.