php包含文件未找到错误

I have a question. I have this script

<?php
$GetPage= "index";
if((isset($_GET["page"])==true) && ($_GET["page"] != "")){
$GetPage = $_GET["page"];
}
?>

But I search on stackoverflow and google. But I can't find it. I want to include a error page when php can't find the file. How can I do that? I'm jut a starter with php.

Ow almost forgoten. I use this to include a part of my site:

<?php include ("include/$GetPage.php"); ?> 

Thanks for reading !

<?php

//file_exists will eliminate the need for any of your other checks.
if(file_exists($_GET["page"])){
    //Set the page to be loaded if it is found on the server
    $GetPage = $_GET["page"];
}else{
    //Show the user a 404 error message
    header("HTTP/1.0 404 Not Found");
    //OR
    //Set the page to be loaded as your custom error page
    $GetPage = "my_error_page.php";
}

//Include the page
include $GetPage;

?>

Are you looking for a 404 redirect? Or just load a custom error page into the document? Select the above based on what you wish to do.

$GetPage = $_GET["page"];  // validate string!!  
if (!file_exists('include/' . $GetPage . '.php')) {
    $GetPage = 'errorPage';
}

first check file is in folder (for injection) , ($file is full path of file.)

$path = "include";//your include folder path
if ( substr(realpath($file) , 0,strlen($path)) != $path || is_dir($file))
        //error file not found

second check file is exist or not

if (!file_exists($file))
//error