Programonic

Before you start

PHP functions.

Hide a property and change it only through methods.
Predict the sample output and explain the state changes.
Complete the practice task and check the boundary cases.

The concept

A PHP class declares properties and methods. private means only code inside the class can touch the property, and $this refers to the current object. Inside methods, members are reached with the -> operator.

new Counter() creates an object with value 0. Each call to increment adds one, and the value() method is the only way outside code can read it.

Code example

PHP

<?php
class Counter {
    private int $value = 0;

    public function increment(): void {
        $this->value++;
    }

    public function value(): int {
        return $this->value;
    }
}

$counter = new Counter();
$counter->increment();
$counter->increment();
echo $counter->value(), "\n";

Output

Example details

Follow the code

Open the walkthrough to inspect the example one step at a time.

Show guided walkthrough

Guided walkthroughReady

1<?php

2class Counter {

3 private int $value = 0;

5 public function increment(): void {

6 $this->value++;

7 }

9 public function value(): int {

10 return $this->value;

11 }

12}

14$counter = new Counter();

15$counter->increment();

16$counter->increment();

17echo $counter->value(), "\n";

Follow the example

Start to inspect the source, values, and output. These prepared steps explain the published sample; grouped loop steps describe multiple iterations.

View the complete dry-run table

Prepared trace of the published program. Some rows group repeated iterations.
What happens	State after step	Output so far
A hidden property that starts at zero.	`—`	—
Create the object.	`value = 0`	—
The first call raises value to 1.	`value = 1`	—
After two calls the value is 2.	`value = 2`	2

Behind the scenes

How PHP processes this code

The PHP runtime executes the script. echo writes to the response or terminal output.

Cost and common mistakes

O(1) per method call.

Reading $counter->value directly is a fatal error because the property is private.
Forgetting $this-> inside a method refers to a local variable, not the property.

Try it yourself

Add a decrement method.

Reveal answer and reasoning

public function decrement(): void { $this->value--; } lowers the value by one.

PHP Classes