1. 通过header()方法来实现，

   最简单的方法，而且对php的版本没什么限制

   <?php

         header('HTTP/1.1 404 Not Found');

         header("status: 404 Not Found");

   ?>

   当服务器设置了404页面后，访问该php就会自动返回404状态及404页面。

2. 使用http_response_code()方法，

     如果你的php版本大于5.4的话，可以使用这个新增的方法

    <?php

        http_response_code(404);

    ?>

    注意：如果你的版本低于5.4，但是你又非得使用http_response_code(),下面是这个方法的代码

<?php

    if (!function_exists('http_response_code')) {
        function http_response_code($code = NULL) {

            if ($code !== NULL) {

                switch ($code) {
                    case 100: $text = 'Continue'; break;
                    case 101: $text = 'Switching Protocols'; break;
                    case 200: $text = 'OK'; break;
                    case 201: $text = 'Created'; break;
                    case 202: $text = 'Accepted'; break;
                    case 203: $text = 'Non-Authoritative Information'; break;
                    case 204: $text = 'No Content'; break;
                    case 205: $text = 'Reset Content'; break;
                    case 206: $text = 'Partial Content'; break;
                    case 300: $text = 'Multiple Choices'; break;
                    case 301: $text = 'Moved Permanently'; break;
                    case 302: $text = 'Moved Temporarily'; break;
                    case 303: $text = 'See Other'; break;
                    case 304: $text = 'Not Modified'; break;
                    case 305: $text = 'Use Proxy'; break;
                    case 400: $text = 'Bad Request'; break;
                    case 401: $text = 'Unauthorized'; break;
                    case 402: $text = 'Payment Required'; break;
                    case 403: $text = 'Forbidden'; break;
                    case 404: $text = 'Not Found'; break;
                    case 405: $text = 'Method Not Allowed'; break;
                    case 406: $text = 'Not Acceptable'; break;
                    case 407: $text = 'Proxy Authentication Required'; break;
                    case 408: $text = 'Request Time-out'; break;
                    case 409: $text = 'Conflict'; break;
                    case 410: $text = 'Gone'; break;
                    case 411: $text = 'Length Required'; break;
                    case 412: $text = 'Precondition Failed'; break;
                    case 413: $text = 'Request Entity Too Large'; break;
                    case 414: $text = 'Request-URI Too Large'; break;
                    case 415: $text = 'Unsupported Media Type'; break;
                    case 500: $text = 'Internal Server Error'; break;
                    case 501: $text = 'Not Implemented'; break;
                    case 502: $text = 'Bad Gateway'; break;
                    case 503: $text = 'Service Unavailable'; break;
                    case 504: $text = 'Gateway Time-out'; break;
                    case 505: $text = 'HTTP Version not supported'; break;
                    default:
                        exit('Unknown http status code "' . htmlentities($code) . '"');
                    break;
                }

                $protocol = (isset($_SERVER['SERVER_PROTOCOL']) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.0');

                header($protocol . ' ' . $code . ' ' . $text);

                $GLOBALS['http_response_code'] = $code;

            } else {

                $code = (isset($GLOBALS['http_response_code']) ? $GLOBALS['http_response_code'] : 200);

            }

            return $code;

        }
    }

?>