SOLID Principles (Overview)

Also Known As

TL;DR

Explanation

SOLID is an acronym for five related OO design principles: Single Responsibility (a class has one reason to change), Open/Closed (open for extension, closed for modification), Liskov Substitution (subclasses substitute for base classes), Interface Segregation (many specific interfaces over one general one), and Dependency Inversion (depend on abstractions, not concretions). Applied together they produce loosely coupled, highly cohesive code that is easy to test and extend. They are guidelines, not rules — apply them where they add value.

Common Misconception

Why It Matters

Common Mistakes

• Treating SOLID as dogma and applying all five principles to simple utility functions — they target complex, change-prone code.
• Confusing Interface Segregation with having one method per interface — it means interfaces should match client needs, not be maximally small.
• Implementing Open/Closed by wrapping everything in strategy objects before there is a real extension need.
• Naming classes *Service or *Manager and calling it SRP — naming does not guarantee single responsibility.

Avoid When

• Small scripts and one-off utilities — applying all five principles adds abstraction overhead with no maintenance benefit.
• Applying SOLID dogmatically — creating interfaces for classes that will never have a second implementation is over-engineering.
• Using SOLID as a checklist rather than guidelines — understanding the problem each principle solves matters more than compliance.

When To Use

• Long-lived codebases that will be extended by multiple developers over time.
• Any class that is becoming hard to test — testability problems usually signal a SOLID violation.
• When adding a new feature requires modifying many existing classes — open-closed principle applies.
• Library and framework code where consumers need to extend behaviour without modifying your source.

Code Examples

class Report {
    public function generate() { /* logic */ }
    public function saveToDatabase() { /* DB */ }
    public function exportToPdf() { /* PDF */ }
    public function sendByEmail() { /* mail */ }
    // Violates SRP, OCP, and DIP simultaneously
}

class Report {
    public function generate(): ReportData { /* pure logic */ }
}
class ReportExporter {
    public function __construct(private ExporterInterface $exporter) {}
    public function export(ReportData $data): void {
        $this->exporter->export($data);
    }
}

References

• ↗ https://en.wikipedia.org/wiki/SOLID

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