Hashing an image from a remote location in PHP - php

Question

Using a PHP script, I want to compare two images. One of the images is located on my server, and one is located on an external website. I have tried to compare the hashes of the two images to each other. Unfortunately, this only works when the two images are saved on my server. How can I make this work?
<?php
$localimage = sha1_file('image.jpg');
$imagelink = file_get_contents('http://www.google.com/image.jpg');
$ext_image = sha1_file($imagelink);
if($localimage == $ext_image){
//Do something
}
?>

Answer 1

If you are using php 5.1+ (which I hope) you can just write :
<?php
$localimage = sha1_file('image.jpg');
$ext_image = sha1_file('http://www.google.com/image.jpg');
if($localimage == $ext_image){
//Do something
}
?>
As sha1_file will work on remote wrappers.
Quote from PHP doc at https://secure.php.net/manual/en/function.sha1-file.php
5.1.0 Changed the function to use the streams API. It means that you can use it with wrappers, like sha1_file('http://example.com/..')

Answer 2

You are not using the sha1_file() properly in the second call.
sha1_file() expects the parameter to be a filename and you are using a memory buffer. So you have 2 options.
First using your current code, save the file to a temp location and use sha1_file()
<?php
$localimage = sha1_file('image.jpg');
$imagelink = file_get_contents('http://www.google.com/image.jpg');
file_put_contents('temp.jpg', $imagelink);
$ext_image = sha1_file('temp.jpg');
if($localimage == $ext_image){
//Do something
}
?>
Or use sha1() instead of sha1_file() on the contents of $imagelink
<?php
$localimage = sha1_file('image.jpg');
$imagelink = file_get_contents('http://www.google.com/image.jpg');
$ext_image = sha1($imagelink);
if($localimage == $ext_image){
//Do something
}
?>
Well actually maybe 3 options, see #Flunch's answer!