How to return a 404 error

Short answer

Set the response code and end the program:

<?php
 
// page.php
 
http_response_code(404);
 
echo 'Page not found';
exit;

A record is not found

This is the answer when the database has no record with the requested number:

<?php
 
// product.php
 
$product = null; // the result of a database query
 
if ($product === null) {
    http_response_code(404);
    require __DIR__ . '/views/404.php';
    exit;
}

For an API

A JSON response:

<?php
 
http_response_code(404);
header('Content-Type: application/json');
 
echo json_encode(['error' => 'Product not found']);

Functions used in this recipe

http_response_code — Sets or returns the response status code.
header — Sends an HTTP response header.
exit — Terminates the program.