PHP 8.4 Asymmetric Visibility: Practical Examples

PHP 8.4 allows properties to have more restrictive write visibility than read visibility.

What It Does and When to Use It

Give callers read access while restricting writes to the declaring class. A minimum platform requirement controls what syntax can be parsed; target-version tests find runtime and extension incompatibilities.

Requirements, Inputs and Expected Behavior

Confirm the runtime, extension, environment variables, and permissions before running the examples in production. Use public private(set) for readable state that only the class can update; syntax needs PHP 8.4+.

Example 1: Basic Operation

Give callers read access while restricting writes to the declaring class.

<?php
final class Profile {
     public private(set) string $status = 'pending';
     public function activate(): void { $this->status = 'active'; }
 }
 $profile = new Profile();
 $profile->activate();
 echo $profile->status;

Example 2: Behavior and Validation

Use a controlled mutator to preserve an invariant.

<?php
final class Counter {
     public private(set) int $value = 0;
     public function increment(): void { ++$this->value; }
 }
 $counter = new Counter(); $counter->increment();
 echo $counter->value;

Practical Example: Application Use

Contrast protected(set) with private(set) for subclasses.

<?php
class Document {
     public protected(set) string $title = 'Draft';
 }
 class EditableDocument extends Document {
     public function rename(string $title): void { $this->title = $title; }
 }
 $d = new EditableDocument(); $d->rename('Final'); echo $d->title;

Important Behavior and Edge Cases

The second example highlights: Use a controlled mutator to preserve an invariant. The practical example adds: Contrast protected(set) with private(set) for subclasses.

Comparison with Related Approaches

Common Beginner Mistakes

Key Takeaways

• PHP 8.4 allows properties to have more restrictive write visibility than read visibility.
• Contrast protected(set) with private(set) for subclasses.
• Do not introduce syntax from a newer minor version before every production host supports it.