PHP Reference Variables and Parameters

Reference parameters can mutate a caller's variable without returning it.

What It Does

Reference parameters can mutate a caller's variable without returning it. The following examples distinguish a normal use case, a boundary case and a practical application.

Requirements and Compatibility

PHP 8.x and a local test environment. Apply input validation, correct error handling and the requirements of any optional libraries.

The code focuses on the core concept. Production implementation may also need routes, validation, persistence, authorization or deployment tooling.

Example 1: Basic Behavior

Start with the smallest meaningful working case.

<?php
$a = 10; $b = &$a; $b = 15; echo $a;

Example 2: Another Case

Examine the edge case and the observable result.

<?php
$first = ['x' => 1]; $copy = $first; $copy['x'] = 9;
echo $first['x']; // Array copy-on-write

Practical Example: Putting It to Use

Reference parameters intentionally mutate the caller's variable.

<?php
function applyDiscount(int &$cents): void { $cents = (int) round($cents * .9); }
$price = 500; applyDiscount($price); echo $price;

Important Behavior and Edge Cases

Document the side effect and avoid keeping accidental foreach references alive.

Comparison with Related Functions or Approaches

Common Beginner Mistakes

Key Takeaways

• Reference parameters can mutate a caller's variable without returning it.
• Document the side effect and avoid keeping accidental foreach references alive.
• Immutable return-value patterns simplify reasoning and tests.

Before adapting these examples, verify the return values, test the failure paths and confirm that your application meets its security and environment requirements.