Constructors and Dependency Injection

Inject a validated dependency into a constructor and store it in a typed readonly property.

The objective is to understand the behavior and operational limits, not only the method name.

What It Does

Inject a validated dependency into a constructor and store it in a typed readonly property.

Prerequisites

Use a dedicated development environment and sample data. Where a snippet uses an injected connection, repository, request value or mailer, supply it from your own application setup.

Example 1: Basic Use

A focused working pattern for this topic. Names such as $pdo, $authContext and $repo represent dependencies provided by the application unless initialized in the snippet.

<?php
final class InvoiceService {
    public function __construct(private readonly PDO $db) {}
    public function count(): int { return (int) $this->db->query('SELECT COUNT(*) FROM invoices')->fetchColumn(); }
}

Example 2: A Second Scenario

A second use case that adds a different operation or data flow specific to this topic.

<?php
interface Repository { public function count(): int; }
final class Dashboard { public function __construct(private Repository $data) {} public function total(): int { return $this->data->count(); } }

Example 3: Practical Checks

A practical validation, failure or deployment concern specific to this topic.

<?php
final class ClockBasedJob {
    public function __construct(private DateTimeImmutable $when) {}
    public function scheduledYear(): int { return (int)$this->when->format('Y'); }
}
echo (new ClockBasedJob(new DateTimeImmutable('2026-01-01')))->scheduledYear();

Important Behavior and Edge Cases

Constructor injection makes dependencies visible and prevents hidden global service creation.

For production use, also account for error handling, data validation, resource limits and environment-specific driver behavior.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• Inject a validated dependency into a constructor and store it in a typed readonly property.
• Constructor injection makes dependencies visible and prevents hidden global service creation.
• Use this workflow as a focused starting point rather than a substitute for application-level tests.

Choose this API or pattern when its exact semantics match your task, and verify the result against the database driver and runtime used by your application.