Writing Testable Classes Without a Framework

Construct small classes whose dependencies arrive through interfaces or constructor parameters.

The examples focus on the operation itself, including the important distinction: Hard-coded service creation couples tests to network or database infrastructure.

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 fake dependency provides a deterministic test without requiring a test framework.

<?php
interface Clock { public function today(): string; }
class FakeClock implements Clock { public function today(): string { return "2026-01-01"; } }
class Badge { public function __construct(private Clock $clock) {}
    public function text(): string { return "Valid " . $this->clock->today(); }
}
echo (new Badge(new FakeClock()))->text();

Output from a successful local PHP 8.4 run:

Valid 2026-01-01

Example 2: Checking Behavior

The fake dependency provides a deterministic test without requiring a test framework. The injected fake supplies a different deterministic date for the test case.

<?php
interface Clock { public function today(): string; }
class FakeClock implements Clock { public function today(): string { return "2026-02-01"; } }
class Badge { public function __construct(private Clock $clock) {}
    public function text(): string { return "Valid " . $this->clock->today(); }
}
echo (new Badge(new FakeClock()))->text();

Output from a successful local PHP 8.4 run:

Valid 2026-02-01

Example 3: Practical Example

The fake dependency provides a deterministic test without requiring a test framework. The injected fake supplies a different deterministic date for the test case.

<?php
interface Clock { public function today(): string; }
class FakeClock implements Clock { public function today(): string { return "2026-03-01"; } }
class Badge { public function __construct(private Clock $clock) {}
    public function text(): string { return "Valid " . $this->clock->today(); }
}
echo (new Badge(new FakeClock()))->text();

Output from a successful local PHP 8.4 run:

Valid 2026-03-01

Important Behavior and Edge Cases

Tests should check externally observable behavior, not private implementation details.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Construct small classes whose dependencies arrive through interfaces or constructor parameters.
• Tests should check externally observable behavior, not private implementation details.
• Hard-coded service creation couples tests to network or database infrastructure.

Use Writing Testable Classes Without a Framework when its documented behavior matches the problem. Test the relevant edge cases in your own runtime before deploying it.