Strategy Pattern

Also Known As

TL;DR

Explanation

The Strategy pattern encapsulates each algorithm in its own class implementing a common interface, making them interchangeable. The context class accepts a strategy via constructor injection and delegates behaviour to it. This eliminates switch/if-else chains that select algorithms (replacing switch smell with polymorphism), makes algorithms independently testable, and follows Open/Closed Principle — new strategies are added without modifying the context. Classic PHP examples: payment processors, sorting algorithms, and report formatters.

Common Misconception

Why It Matters

Common Mistakes

• Strategies that depend on the context object — they should be independent algorithms.
• Too many strategies for trivial variations — a simple parameter is cleaner than a strategy class.
• Not using interfaces for strategies — callers become coupled to concrete strategy types.
• Forgetting to inject the strategy — instantiating it inside the context defeats the purpose.

Code Examples

function sortUsers(array $users, string $by): array {
    if ($by === 'name') {
        usort($users, fn($a,$b) => strcmp($a->name, $b->name));
    } elseif ($by === 'age') {
        usort($users, fn($a,$b) => $a->age <=> $b->age);
    } elseif ($by === 'score') {
        usort($users, fn($a,$b) => $b->score <=> $a->score);
    } // must edit this function for every new sort
    return $users;
}

interface SortStrategy {
    public function compare(User $a, User $b): int;
}

class SortByName  implements SortStrategy { public function compare(User $a, User $b): int { return strcmp($a->name, $b->name); } }
class SortByAge   implements SortStrategy { public function compare(User $a, User $b): int { return $a->age <=> $b->age; } }
class SortByScore implements SortStrategy { public function compare(User $a, User $b): int { return $b->score <=> $a->score; } }

function sortUsers(array $users, SortStrategy $strategy): array {
    usort($users, $strategy->compare(...));
    return $users;
}

References

• ↗ https://refactoring.guru/design-patterns/strategy

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