PHP fopen: failed to open stream: Success + how to save fetched content to a file?

I have programmed a "proxy.php" script (listed below), which would fetch an image specified in the ?img= parameter and print it to the STDOUT. This is needed for my Flash app to circumvent a missing crossdomain.xml at some sites.

It works, but I have 3 questions please. Also I'm coming to PHP from Perl and still have many gaps in my PHP-knowledge (but I realize, that stream_context_create and fpassthru probably use bucket brigades).

1) In my callback() function, how can I print开发者_如何学JAVA debugging messages to PHP log? (it is redirected to /var/log/messages at my CentOS machine)

2) Why do I get the error message failed to open stream: Success, do I miss a case in the callback() maybe?

PHP Warning:  fopen() [<a href='function.fopen'>function.fopen</a>]: php_network_getaddresses: getaddrinfo failed: Name or service not known in /var/www/html/proxy.php on line 19
PHP Warning:  fopen(http://i136.odnoklassniki.ru/getImage?photoId=154105499212&amp;photoType=0) [<a href='function.fopen'>function.fopen</a>]: failed to open stream: Success in /var/www/html/proxy.php on line 19

3) Because my script is often called with the same image URL as parameter, I would like to extend it so that it saves the fetched file in a dir on the 1st call. And on the 1st and subsequent calls it should serve that cached file to STDOUT. Do you have a suggestion, how to make it in a memory-conserving way? I.e. I don't want to read the whole file at once with get_file_contents()

<?php

define('MAX_SIZE', 1024 * 1024);

$img = urldecode($_GET['img']);
if (strpos($img, '..') !== FALSE)
        exit('Wrong URL: ' . $img);

$opts = array(
        'http'=>array(
                'method' => 'GET'
        )
);

$ctx = stream_context_create($opts);
stream_context_set_params($ctx, array('notification' => 'callback'));
$fh = fopen($img, 'r', FALSE, $ctx);
if ($fh) {
        fpassthru($fh);
        fclose($fh);
}

function callback($code, $severity, $message, $message_code, $bytes_transferred, $bytes_total) {
        if ($code == STREAM_NOTIFY_PROGRESS && $bytes_transferred > MAX_SIZE)
                exit('File is too big: ' . $bytes_transferred);

        if ($code == STREAM_NOTIFY_FILE_SIZE_IS)
                if ($bytes_total > MAX_SIZE)
                        exit('File is too big: ' . $bytes_total);
                else
                        header('Content-Length: ' . $bytes_total);

        if ($code == STREAM_NOTIFY_MIME_TYPE_IS) {
                if (stripos($message, 'image/gif') !== FALSE ||
                    stripos($message, 'image/png') !== FALSE ||
                    stripos($message, 'image/jpg') !== FALSE ||
                    stripos($message, 'image/jpeg') !== FALSE) {
                        header('Content-Type: ' . $message);
                } else {
                        exit('File is not image: ' . $mime);
                }
        }
}

?>

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1. Because:

   sviss@sviss:~$ host i136.odnoklassniki.ru
   Host i136.odnoklassniki.ru not found: 3(NXDOMAIN)