PHP Static Properties

Static Properties & Class State

Static properties are class variables shared across all instances of a class. Unlike instance properties, static properties belong to the class definition itself and retain their value state across all object instances.

Static Property Access

• Inside Class: self::$propertyName or static::$propertyName
• Outside Class: ClassName::$propertyName

flowchart TD
    A["Static Property (User::$instanceCount)"] --> B["Object Instance 1"]
    A --> C["Object Instance 2"]
    A --> D["Object Instance 3"]
    B & C & D -->|All instances read & update SAME memory state| A

Practical Code Example

<?php
declare(strict_types=1);

class DatabaseConnectionPool
{
    // Shared Static Property tracking total active instances
    public static int $activeConnections = 0;

    public function __construct(public string $connectionName)
    {
        // Increment shared class counter
        self::$activeConnections++;
    }

    public function close(): void
    {
        if (self::$activeConnections > 0) {
            self::$activeConnections--;
        }
    }

    public function __destruct()
    {
        // Decrement on garbage collection
    }
}

$conn1 = new DatabaseConnectionPool("Main_DB");
$conn2 = new DatabaseConnectionPool("Analytics_DB");
$conn3 = new DatabaseConnectionPool("Cache_DB");

echo "Active Connections Count: " . DatabaseConnectionPool::$activeConnections . "
"; // Outputs 3

$conn1->close();
echo "Active Connections After 1 Close: " . DatabaseConnectionPool::$activeConnections . "
"; // Outputs 2

Best Practices

• Use Static Properties for Shared Global Counters & Caches: Ideal for tracking instance counters or in-memory singleton instances.
• Include $ Sign When Accessing Static Properties: Write ClassName::$prop (with $), unlike static methods or constants.
• Watch Out for Global State Contamination: Mutating global static properties can cause hidden side-effect bugs across test cases.

Self-Check Challenge

Write a static property declaration public static int $counter = 0; inside a class VisitorCounter and increment it inside __construct()!