PHP Typed Class Constants

A typed class constant declares the type of a constant value and validates compatible overrides.

The examples focus on the operation itself, including the important distinction: readonly properties apply to instance data; typed constants are immutable class-level declarations.

What It Does

Use the examples below to observe the behavior on a small dataset before extending it to application code.

Before You Start

Read the input requirements before applying this pattern to production data.

Example 1: Basic Use

The constant type is enforced by PHP, independently of property visibility. Requires PHP 8.3+.

<?php
class ResponseCodes {
    public const int OK = 200;
    public const string LABEL = "success";
}
echo ResponseCodes::OK;

Output from a successful local PHP 8.4 run:

200

Example 2: Checking Behavior

The constant type is enforced by PHP, independently of property visibility. Requires PHP 8.3+.

<?php
class ResponseCodes {
    public const int OK = 201;
    public const string LABEL = "success";
}
echo ResponseCodes::OK;

Output from a successful local PHP 8.4 run:

201

Example 3: Practical Example

The constant type is enforced by PHP, independently of property visibility. Requires PHP 8.3+.

<?php
class ResponseCodes {
    public const int OK = 202;
    public const string LABEL = "success";
}
echo ResponseCodes::OK;

Output from a successful local PHP 8.4 run:

202

Important Behavior and Edge Cases

Available from PHP 8.3; a constant type declaration is distinct from a property type.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• A typed class constant declares the type of a constant value and validates compatible overrides.
• Available from PHP 8.3; a constant type declaration is distinct from a property type.
• readonly properties apply to instance data; typed constants are immutable class-level declarations.

Use PHP Typed Class Constants when its documented behavior matches the problem. Test the relevant edge cases in your own runtime before deploying it.