Which OOP principle is considered the most fundamental for building reliable objects?

The most fundamental principle of OOP for building reliable objects is considered to be encapsulation.

Encapsulation allows hiding the internal implementation details of an object and providing access to data and behavior only through a defined interface. This protects the object from incorrect usage and makes maintenance and modification of the code easier.

In PHP, encapsulation is implemented using access modifiers:

• private — accessible only within the class
• protected — accessible within the class and its subclasses
• public — accessible from anywhere

Example:

class User {
    private $password;

    public function setPassword($pass) {
        // you can add complexity checks here
        $this->password = password_hash($pass, PASSWORD_DEFAULT);
    }

    public function verifyPassword($pass) {
        return password_verify($pass, $this->password);
    }
}

Here, the password is hidden from direct access, which increases the security and reliability of the object.