Dependency Injection in Tests

Testing with Dependency Injection

// Hard-coded dependency — HARD TO TEST
class ReportGenerator {
    public function generate(): array {
        $pdo = new PDO("mysql:host=prod-db;dbname=app", "user", "pass"); // hardcoded!
        return $pdo->query("SELECT * FROM sales")->fetchAll();
    }
}

// Injected dependency — EASY TO TEST
class ReportGenerator {
    public function __construct(private PDO $pdo) {}

    public function generate(): array {
        return $this->pdo->query("SELECT * FROM sales")->fetchAll();
    }
}

// Test with in-memory SQLite
class ReportGeneratorTest extends TestCase {
    public function testGenerate(): void {
        $pdo = new PDO("sqlite::memory:");
        $pdo->exec("CREATE TABLE sales (id INT, amount FLOAT)");
        $pdo->exec("INSERT INTO sales VALUES (1, 99.99)");

        $report = (new ReportGenerator($pdo))->generate();

        $this->assertCount(1, $report);
        $this->assertEquals(99.99, $report[0]["amount"]);
    }
}