PHP Static Properties and Methods Explained

Normal properties and methods belong to an object. Static ones belong to the class itself. You use them with :: and no new.

index.php

PHP

<?php
class MathHelper
{
    public static function square(int $n): int
    {
        return $n * $n;
    }
}

echo MathHelper::square(7);

Output

OUTPUT

Static properties

A static property is shared by all objects of the class. There is only one copy.

index.php

PHP

<?php
class Visitor
{
    public static int $count = 0;

    public function __construct()
    {
        self::$count++;
    }
}

new Visitor();
new Visitor();
new Visitor();

echo Visitor::$count;

Output

OUTPUT

Inside the class, write self::$count. From outside, write Visitor::$count.

No $this in static methods

A static method is not tied to an object, so $this does not exist there. It can use only static properties, other static methods and constants.

index.php

PHP

<?php
class Config
{
    private static array $values = [];

    public static function set(string $key, string $value): void
    {
        self::$values[$key] = $value;
    }

    public static function get(string $key, string $default = ""): string
    {
        return self::$values[$key] ?? $default;
    }
}

Config::set("site", "Code Corner");

echo Config::get("site") . "\n";
echo Config::get("missing", "n/a");

Output

OUTPUT

1
2

Code Corner
n/a

self vs static

self:: always means the class where the code is written. static:: means the class that was actually called (late static binding). It matters with inheritance.

index.php

PHP

<?php
class Model
{
    public static function create(): static
    {
        return new static();
    }

    public static function name(): string
    {
        return static::class;
    }
}

class User extends Model {}

echo User::name() . "\n";
echo get_class(User::create());

Output

OUTPUT

1
2

User
User

If create() used new self(), it would build a Model, not a User.

Static factory methods

A static method is a good place for a named constructor.

index.php

PHP

<?php
class Temperature
{
    private function __construct(public readonly float $celsius) {}

    public static function fromFahrenheit(float $f): self
    {
        return new self(($f - 32) / 1.8);
    }
}

echo round(Temperature::fromFahrenheit(212)->celsius);

Output

OUTPUT

When to avoid statics

A static property is global state. Any code can change it, so bugs are hard to find.
Static methods are hard to replace in tests.
A class full of static methods is just a bag of functions with a name.

Utility functions (MathHelper::square) and factory methods are fine. For services that hold state (a database connection, a logger), use normal objects and pass them in. See Dependency Injection.

info

Not the same as static variables

static $x inside a function (see Static Variables) keeps a value between calls. Same keyword, different feature.