PHP headers_list Function

last modified April 4, 2025

The PHP headers_list function retrieves the list of response headers to be sent to the client. It’s useful for debugging and header inspection.

Basic Definition

headers_list returns an array of headers prepared for sending. These include both PHP-generated and manually set headers using header().

Syntax: headers_list(): array. Returns a numerically indexed array. Headers are only available after output buffering starts or before output.

Basic Header Inspection

This example shows how to retrieve and display all response headers.

basic_headers.php

<?php

declare(strict_types=1);

header(‘Content-Type: text/html; charset=utf-8’); header(‘X-Powered-By: PHP/8.1’);

$headers = headers_list(); print_r($headers);

This displays all headers set for the response, including Content-Type. The output shows both system-generated and custom headers.

Checking for Specific Headers

This demonstrates how to check if a specific header has been set.

check_specific_header.php

<?php

declare(strict_types=1);

header(‘Cache-Control: no-cache’);

function has_header(string $name): bool { foreach (headers_list() as $header) { if (stripos($header, $name) === 0) { return true; } } return false; }

echo has_header(‘Cache-Control’) ? ‘Cache header set’ : ‘No cache header’;

This checks if Cache-Control header exists in the response headers. The function searches for headers starting with the specified name.

Header Modification Detection

This example shows how to detect if headers have been modified.

header_modification.php

<?php

declare(strict_types=1);

$initial_headers = headers_list();

header(‘X-Custom-Header: Value’); header_remove(‘X-Powered-By’);

$current_headers = headers_list(); $modified = $initial_headers != $current_headers;

echo $modified ? ‘Headers changed’ : ‘Headers unchanged’;

This compares headers before and after modifications to detect changes. Useful for middleware that needs to verify header modifications.

Content-Type Validation

This shows how to validate the Content-Type header before output.

content_type_check.php

<?php

declare(strict_types=1);

function validate_content_type(string $expected): bool { foreach (headers_list() as $header) { if (stripos($header, ‘Content-Type:’) === 0) { return stripos($header, $expected) !== false; } } return false; }

header(‘Content-Type: application/json’); echo validate_content_type(‘json’) ? ‘Valid’ : ‘Invalid’;

This ensures the response Content-Type matches expectations before sending. Particularly useful in API implementations requiring specific formats.

Header Debugging Tool

This example creates a simple header debugging tool for development.

header_debugger.php

<?php

declare(strict_types=1);

header(‘X-Debug: Enabled’); header(‘Cache-Control: no-store’);

function debug_headers(): string { $output = “<h3>Response Headers</h3><ul>”; foreach (headers_list() as $header) { $output .= “<li>” . htmlspecialchars($header) . “</li>”; } return $output . “</ul>”; }

echo debug_headers();

This creates an HTML list of all response headers for debugging purposes. Helpful during development to verify header configuration.

Best Practices

• Timing: Call before any output to get complete headers

• Performance: Avoid excessive calls in production

• Security: Don’t expose sensitive headers to end users

• Testing: Use for unit testing header configurations

Source

PHP headers_list Documentation

This tutorial covered the PHP headers_list function with practical examples for header inspection, validation, and debugging in PHP applications.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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