Réduire la résolution d'image en php

<?php
// The file you are resizing
$file = $_GET['img'];

if(file_exists("./map/".$file))
{
	if(!file_exists("./map/thumbs/$file"))
	{
		// This sets it to a .jpg, but you can change this to png or gif
		//header('Content-type: image/jpeg');
		// Setting the resize parameters
		list($width, $height) = getimagesize("./map/".$file);
		$modwidth = 278;	//$width * $size;
		$modheight = 155;	//$height * $size;

		// Resizing the Image
		$tn = imagecreatetruecolor($modwidth, $modheight);
		$image = imagecreatefromjpeg("./map/".$file);
		imagecopyresampled($tn, $image, 0, 0, 0, 0, $modwidth, $modheight, $width, $height);

		// Outputting a .jpg, you can make this gif or png if you want
		//notice we set the quality (third value) to 100
		//imagejpeg($tn, null, 80);
		imagejpeg($tn, "./map/thumbs/$file", 80);
		imagedestroy($tn);
	}
	header("Content-type: image/jpeg");
	readfile("./map/thumbs/$file");
}
else
{
	die($_GET['img']." not found.");
}
?>