How do you decide whether to use an interface or an abstract class when designing a set of related classes that share common behavior?

You generally choose an interface when you want to define a pure contract without providing any shared implementation, especially when unrelated classes need to guarantee the same set of capabilities, and you choose an abstract class when you have genuinely shared implementation logic that multiple related subclasses can reuse as is, combined with certain behaviors that must still be customized by each specific subclass, keeping in mind that a class can implement multiple interfaces but can only extend a single abstract class.

// Interface: unrelated classes sharing a capability
interface Loggable { public function getLogMessage(): string; }

// Abstract class: related classes sharing implementation
abstract class PaymentMethod {
    public function logTransaction() { /* shared logic */ }
    abstract public function process(float $amount): bool;
}