<?php 

//исходный файл 1920 * 1200, для примера

$file = $_GET['file'];  

list($width, $height, $type) = getimagesize($file);

$slashPos = strrpos($file,'_');

$fileName = substr($file,$slashPos);

$fileName = str_replace("_", "1", $fileName);

$str="Content-Disposition: attachment; filename=".$fileName;

header($str); 

header("Content-type: $type");

$new_height = 768;

$k = $height / $new_height;

$new_width = $width / $k;

$image_p = imagecreatetruecolor($new_width, $new_height);

$image = imagecreatefromjpeg($file);

imagecopyresampled($image_p, $image, 0, 0, 0, 0, $new_width, $new_height, $width, $height);

imagejpeg($image_p, null, 100);

$file = $image_p; 

// создана картинка с заданной высотой, ширина чуток больше






$end_width = 1024;

$end_height = 768;

list($width2, $height2, $type) = getimagesize($file); // 1228 * 768

$slashPos = strrpos($file,'_');

$fileName = substr($file,$slashPos);

$fileName = str_replace("_", "1", $fileName);

$str="Content-Disposition: attachment; filename=".$fileName;

header($str); 

header("Content-type: $type");

$razn = $width2 - $end_width; // 234 - разница между исходным и требуемым по ширине

$razn = $razn / 2; // 117 - разница в ширине с одной стороны

$image_p = imagecreatetruecolor($new_width, $new_height);

$image = imagecreatefromjpeg($file);

imagecopyresampled($image_p, $image, 0, 0, $razn, 0, $end_width, $end_height, $width2, $height2);

imagejpeg($image_p, null, 80);

$file = $image_p; 

$str="Content-Disposition: attachment; filename=".$fileName;

header($str); 

header("Content-type: application/octet-stream"); 

echo file_get_contents($file); 

?>