file_exists（$ img）和@gegreize（$ img）总是返回false

I have a script that lets users upload an own header image for specific pages of my website. To check if the user has uploaded an image I use file_exists($img). But the function always returns false.

I've also tried using @getimagesize($img) which didn't work either. When I use echo "<img src='".$img."'> though, the picture shows without problem.

Maybe there's a mistake in my code, I don't know. Or maybe it has something to do with security issues? I tried using different files and chmod() though. Please help.

Here is my code:

define('PATH', '/root/');
define('LAYOUT', 'header1');

//This is always returning false
if (defined('LAYOUT') && (file_exists(PATH.'img/header/'.LAYOUT.'.png') OR 
    file_exists(PATH.'img/header/'.LAYOUT.'.jpg'))) {
    echo "true";
} else {
    echo "false";
}

//This is also returning false
if (defined('LAYOUT') && (@getimagesize($img)) {
    echo "true";
} else {
    echo "false";
}

//This is showing the picture
echo "<img style='width: 100%;' src='".PATH."img/header/".LAYOUT.".png'>";

EDIT: I solved it by using file_exists(__DIR__ . '\\www\\img\\header\\'.LAYOUT.'.png'). Thank you!