Return a PHP page as an image

This should work. It may be slower.

$img = imagecreatefromjpeg($filename);
header("Content-Type: image/jpg");
imagejpeg($img);
imagedestroy($img);

The PHP Manual has this example:

<?php
// open the file in a binary mode
$name = './img/ok.png';
$fp = fopen($name, 'rb');

// send the right headers
header("Content-Type: image/png");
header("Content-Length: " . filesize($name));

// dump the picture and stop the script
fpassthru($fp);
exit;
?>

The important points is that you must send a Content-Type header. Also, you must be careful not include any extra white space (like newlines) in your file before or after the <?php ... ?> tags.

As suggested in the comments, you can avoid the danger of extra white space at the end of your script by omitting the ?> tag:

<?php
$name = './img/ok.png';
$fp = fopen($name, 'rb');

header("Content-Type: image/png");
header("Content-Length: " . filesize($name));

fpassthru($fp);

You still need to carefully avoid white space at the top of the script. One particularly tricky form of white space is a UTF-8 BOM. To avoid that, make sure to save your script as "ANSI" (Notepad) or "ASCII" or "UTF-8 without signature" (Emacs) or similar.

I feel like we can make this code a little bit easier by just getting the mime type from $image_info:

$file_out = "myDirectory/myImage.gif"; // The image to return

if (file_exists($file_out)) {

   $image_info = getimagesize($file_out);

   //Set the content-type header as appropriate
   header('Content-Type: ' . $image_info['mime']);

   //Set the content-length header
   header('Content-Length: ' . filesize($file_out));

   //Write the image bytes to the client
   readfile($file_out);
}
else { // Image file not found

    header($_SERVER["SERVER_PROTOCOL"] . " 404 Not Found");

}

With this solution any type of image can be processed but it is just another option. Thanks ban-geoengineering for your contribution.