Chapter 04 · OOP Foundations

Classes & Objects

This is the most important chapter in the whole course. Once you truly understand the difference between a class and an object, everything else in OOP becomes easy. Take it slowly and type every example.

The cookie cutter and the cookies

Your first class

student.php<?php
class Student {
    // Properties = the data each student will carry (variables that live inside the class)
    public string $name = "";
    public string $program = "";

    // Method = a function that lives inside the class (a skill every student has)
    public function introduce(): string {
        return "Hi, I am $this->name and I study $this->program.";
    }
}

Reading it piece by piece:

• class Student { ... } — "Here is the design for a thing called Student." Class names start with a Capital Letter by convention.
• Properties ($name, $program) — variables that belong to the class. Every student made from this design will have its own copy of them.
• Methods (introduce()) — functions that belong to the class. They describe what a student can do.
• public — means "anyone may access this". (Chapter 06 introduces the private option.)

Making objects with new

$s1 = new Student();   // cut the first cookie
$s2 = new Student();   // cut a second, completely separate cookie

$s1->name = "Chanda";
$s1->program = "BSc Computing";

$s2->name = "Mutinta";
$s2->program = "LLB";

echo $s1->introduce();
echo "<br>";
echo $s2->introduce();

Two new symbols to learn — and they're the last new symbols for a while:

• new Student() — "build me a fresh object from the Student design."
• -> — the object operator, a little arrow. Read it as "'s" (apostrophe-s). $s1->name is "s1's name". $s1->introduce() is "s1's introduce skill — go!".

Notice that changing $s1's name did nothing to $s2. Each object has its own copy of every property — separate cookies.

The mysterious $this

Look inside introduce() again: it says $this->name. What is $this?

class Student {
    public string $name = "";

    public function shout(): string {
        // "$this->name" = "MY OWN name, whoever I am"
        return strtoupper($this->name) . " IS HERE!";
    }
}

$a = new Student();
$a->name = "Bwalya";
echo $a->shout();     // $this was $a

Methods can use other methods

Objects can call their own skills using $this too. Watch a slightly bigger, realistic example:

class BankAccount {
    public string $owner = "";
    public float $balance = 0;

    public function deposit(float $amount): void {
        $this->balance += $amount;
    }

    public function withdraw(float $amount): string {
        if ($amount > $this->balance) {
            return "Sorry $this->owner, insufficient funds.";
        }
        $this->balance -= $amount;
        return "Done. " . $this->statement();   // calling my own other method!
    }

    public function statement(): string {
        return "$this->owner's balance is K" . number_format($this->balance, 2);
    }
}

$acc = new BankAccount();
$acc->owner = "Thandiwe";
$acc->deposit(1000);
echo $acc->withdraw(250);
echo "<br>";
echo $acc->withdraw(5000);

See how natural it reads? The account protects itself — the withdraw method refuses to go below zero. Data and rules travelling together: that's OOP working for you already.

Objects in arrays — a class register

Objects are values like any other, so you can put them in arrays and loop over them:

$class = [];

$s = new Student(); $s->name = "Chanda";   $class[] = $s;
$s = new Student(); $s->name = "Mutinta";  $class[] = $s;
$s = new Student(); $s->name = "Bwalya";   $class[] = $s;

foreach ($class as $student) {
    echo $student->name . "<br>";
}

This exact pattern — "a list of objects, looped over to build HTML" — is how every real application displays a table of students, products or messages. Remember it.

<?php
class Phone {
    public string $brand = "";
    public int $batteryLevel = 100;

    public function use30Minutes(): void {
        $this->batteryLevel -= 10;
    }

    public function status(): string {
        return "$this->brand battery: $this->batteryLevel%";
    }
}

$p = new Phone();
$p->brand = "Samsung";
$p->use30Minutes();
$p->use30Minutes();
echo $p->status();