basic PHP image proxy (that works ... )

imageProxy.php

<?php

$url = isset($_GET['url']) ? $_GET['url'] : null;

if (!$url) {
    die('Please, inform URL');
}

$imgInfo = getimagesize( $url );

if ($imgInfo === false) {
    die('Could not retrieve information');
}

if (stripos($imgInfo['mime'], 'image/') === false) {
    die('Invalid image file');
}

header("Content-type: ".$imgInfo['mime']);
readfile( $url );

Line 11 may return some warnings in current PHP versions:
PHP Warning: Trying to access array offset on value of type bool in imageProxy.php on line 11
PHP Deprecated: stripos(): Passing null to parameter #1 ($haystack) of type string is deprecated in imageProxy.php on line 11

@Looper1984 wrong/unreadable url may cause it. You could fix by checking getimagesize as bellow:

if ($imgInfo === false) {
    die('Could not retrieve information');
}

@carloscarucce that fixed it, thank you :)

I may still get a warning when getimagesize fails, though, because the error handling comes after it. But of course all is taken care of, so I simply suppress the warning like this:
$imgInfo = @getimagesize( $url );

@Looper1984 glad it helped. I would not leave error messages in a production environment, through.

You can hide it by changing a directive in your php.ini: display_errors = off. Or programmatically with ini_set("display_errors", 0);

References:
Article: https://www.a2hosting.com/kb/developer-corner/php/using-php.ini-directives/php-error-messages/
S.O. Thread: https://stackoverflow.com/a/332206/3435728 (accepted answer teaches on how to enable logging too)

@carloscarucce right, that's much safer and exactly what I'm doing. :)