<h2>What Is SOLID?</h2><p>SOLID is a set of five design principles that help developers write <strong>maintainable, flexible, and understandable</strong> code. Let's break each one down.</p><h3>S — Single Responsibility</h3><p>A class should have <em>one reason to change</em>. If your <code>UserController</code> handles authentication, profile updates, and email notifications, it has too many responsibilities.</p><h3>O — Open/Closed</h3><p>Classes should be <strong>open for extension</strong> but <strong>closed for modification</strong>. Use interfaces and abstract classes to allow new behavior without changing existing code.</p><h3>L — Liskov Substitution</h3><p>Subtypes must be substitutable for their base types without breaking the program. If <code>Square extends Rectangle</code>, setting width shouldn't change height.</p><h3>I — Interface Segregation</h3><p>Don't force classes to implement interfaces they don't use:</p><ul><li><em>Bad:</em> One massive <code>WorkerInterface</code> with 20 methods</li><li><em>Good:</em> Small, focused interfaces like <code>CanSendEmail</code> and <code>CanGenerateReport</code></li></ul><h3>D — Dependency Inversion</h3><p>Depend on <em>abstractions</em>, not <em>concretions</em>:</p><pre><code>// Good: depends on interface\npublic function __construct(private PaymentGateway $gateway) {}\n\n// Bad: depends on concrete class\npublic function __construct(private StripeGateway $gateway) {}</code></pre><blockquote><p>SOLID principles are guidelines, not laws. Apply them where they reduce complexity — don't over-engineer simple code to satisfy a principle.</p></blockquote>