Chapter 05 · OOP Foundations

Constructors

In the last chapter, creating a student took four lines: make it, set the name, set the program, set the balance. Real developers do it in one line — thanks to a special method called the constructor.

The problem

$s = new Student();
$s->name = "Chanda";
$s->program = "BSc Computing";
$s->balance = 5000;

Annoying to type — and dangerous. What if you forget to set the name? You'd have a half-built "ghost student" wandering through your program, causing errors later. We want a rule: a student cannot exist without a name.

Meet __construct

A constructor is just a method with the special name __construct (two underscores). PHP calls it automatically whenever you say new:

<?php
class Student {
    public string $name;
    public string $program;
    public float $balance;

    public function __construct(string $name, string $program, float $balance) {
        $this->name    = $name;
        $this->program = $program;
        $this->balance = $balance;
        echo "($name has been registered!)<br>";
    }

    public function introduce(): string {
        return "I am $this->name, studying $this->program.";
    }
}

// Now creating a student is ONE line:
$s1 = new Student("Chanda", "BSc Computing", 5000);
$s2 = new Student("Mutinta", "LLB", 7200);

echo $s1->introduce();

Follow the journey of the word "Chanda":

1. new Student("Chanda", ...) — you hand "Chanda" to the registration desk.
2. It lands in the parameter $name inside __construct.
3. $this->name = $name; — "put the name I was given into my own name property." Left side: the object's permanent property. Right side: the temporary parameter.

Default values — optional details

Constructor parameters can have defaults, just like normal functions:

class Student {
    public function __construct(
        public string $name,
        public string $program,
        public float $balance = 0    // optional; 0 if not provided
    ) {}
}

$a = new Student("Bwalya", "BBA");          // balance becomes 0
$b = new Student("Thandiwe", "LLB", 2500);  // balance is 2500

Wait — did you notice something amazing in that example? Look closer...

Promoted properties — the modern shortcut

Since PHP 8, you can declare properties directly inside the constructor's brackets by adding public (or private) before each parameter. PHP then creates the property and assigns the value for you. These two classes are identical:

// The long, classic way
class Course {
    public string $code;
    public string $title;

    public function __construct(string $code, string $title) {
        $this->code  = $code;
        $this->title = $title;
    }
}

// The modern shortcut — "constructor property promotion"
class Course {
    public function __construct(
        public string $code,
        public string $title
    ) {}
}

Both let you write new Course("COMP101", "Intro to Programming"). You will meet both styles in real projects, so learn to read both. In this course we'll mostly use the modern shortcut because it's shorter and clearer.

Constructors can do work, not just store

The registration desk doesn't only file your form — it can also compute things, check rules, and prepare the object for life:

class Invoice {
    public string $number;
    public float $total;

    public function __construct(public string $studentName, public float $amount) {
        $this->total  = $amount * 1.16;                    // add 16% VAT automatically
        $this->number = "INV-" . strtoupper(uniqid());     // generate a unique invoice number
    }
}

$inv = new Invoice("Chanda", 1000);
echo "$inv->number for $inv->studentName: K" . number_format($inv->total, 2);

__destruct — the goodbye method

The rarely-used twin of the constructor: __destruct() runs automatically when an object is destroyed (usually when the script ends). It's occasionally used for cleanup, like closing files. You mostly just need to know it exists:

class Visitor {
    public function __construct(public string $name) {
        echo "$this->name entered.<br>";
    }
    public function __destruct() {
        echo "$this->name left. Goodbye!<br>";
    }
}

$v = new Visitor("Chanda");
echo "...doing some work...<br>";

<?php
class Book {
    public function __construct(
        public string $title,
        public string $author,
        public int $pages = 100
    ) {}

    public function summary(): string {
        return "$this->title by $this->author ($this->pages pages)";
    }
}

$b1 = new Book("1984", "George Orwell", 328);
$b2 = new Book("Short Stories", "N. Mulenga");   // uses default 100

echo $b1->summary() . "<br>";
echo $b2->summary();