Purpose of PHP constructors

Question

I am working with classes and object class structure, but not at a complex level – just classes and functions, then, in one place, instantiation.

As to __const

Answer 1

The constructor is run at the time you instantiate an instance of your class. So if you have a class Person:

class Person {

    public $name = 'Bob'; // this is initialization
    public $age;

    public function __construct($name = '') {
        if (!empty($name)) {
            $this->name = $name;
        }
    }

    public function introduce() {
        echo "I'm {$this->name} and I'm {$this->age} years old\n";
    }

    public function __destruct() {
        echo "Bye for now\n";
    }
}

To demonstrate:

$person = new Person;
$person->age = 20;
$person->introduce();

// I'm Bob and I'm 20 years old
// Bye for now

We can override the default value set with initialization via the constructor argument:

$person = new Person('Fred');
$person->age = 20;
$person->introduce();

// if there are no other references to $person and 
// unset($person) is called, the script ends 
// or exit() is called __destruct() runs
unset($person);

// I'm Fred and I'm 20 years old
// Bye for now

Hopefully that helps demonstrate where the constructor and destructor are called, what are they useful for?

1. __construct() can default class members with resources or more complex data structures.
2. __destruct() can free resources like file and database handles.
3. The constructor is often used for class composition or constructor injection of required dependencies.