Handling CORS for PHP APIs

Return explicit cross-origin response permissions and handle OPTIONS preflight requests. The examples separate the essential operation from the conditions a production implementation must enforce.

What It Does

Return explicit cross-origin response permissions and handle OPTIONS preflight requests.

Prerequisites

Use a dedicated development environment with test accounts and records. Separate authorization, validation and error handling from response formatting; never copy demonstration-only secrets into production.

Example 1: Core Behavior

The first example isolates the core operation so its result is easy to understand.

<?php
$origin=$_SERVER['HTTP_ORIGIN']??''; $allowed=['https://app.example.test'];
if (in_array($origin,$allowed,true)) {header('Access-Control-Allow-Origin: '.$origin); header('Vary: Origin');}
echo 'CORS policy checked';

Example 2: Alternative or Failure Case

The second example demonstrates an alternative outcome, rule or failure condition.

<?php
if (($_SERVER['REQUEST_METHOD']??'')==='OPTIONS') {header('Access-Control-Allow-Methods: GET, POST');header('Access-Control-Allow-Headers: Content-Type, Authorization');http_response_code(204);exit;}
echo 'Regular request';

Practical Example: Applying the Rule

The practical example shows how to carry the same constraint into application code.

<?php
$origin=$_SERVER['HTTP_ORIGIN']??'';
if ($origin==='https://app.example.test') {header('Access-Control-Allow-Origin: '.$origin);header('Access-Control-Allow-Credentials: true');header('Vary: Origin');}
echo 'Explicit credential origin';

Important Behavior and Edge Cases

Never combine Access-Control-Allow-Origin: * with credentialed browser requests.

Comparison with Related Functions or Patterns

Common Beginner Mistakes

Key Takeaways

• Return explicit cross-origin response permissions and handle OPTIONS preflight requests.
• Never combine Access-Control-Allow-Origin: * with credentialed browser requests.
• CORS controls browser reads; it is not authentication or CSRF protection.

Select this API or pattern based on its documented behavior and the trust boundaries in your own application, and test all external integrations against their actual deployments.