A couple of notes regarding the examples here (I'm using PHP 7.0.18):

1) fopen() did not provide STREAM_NOTIFY_PROGRESS for me but `file_get_contents()` worked fine.

2) The bytes transferred when transferring a zip file seems to always be shy by 8192 (8k).  This varies depending on filetype and also breaks STREAM_NOTIFY_PROGRESS when the filesize is below 8k.  I modified the function and it's now displaying the correct amount transferred and percentage downloaded on zip files above 8k.  I'm not sure why the callback is so inconsistent but hopefully this is helpful to someone.  This was also modified to only display progress:

<?php

$ctx = stream_context_create();
stream_context_set_params($ctx, array("notification" => "stream_notification_callback"));
$fileData = @file_get_contents('http://example.com/test.zip',false,$ctx);

function stream_notification_callback($notification_code, $severity, $message, $message_code, $bytes_transferred, $bytes_max) {
    static $filesize = null;
    switch($notification_code) {
    case STREAM_NOTIFY_FILE_SIZE_IS:
        $filesize = $bytes_max;
        break;
    case STREAM_NOTIFY_CONNECT:
        echo "Connected...\n";
        break;
    case STREAM_NOTIFY_PROGRESS:
        if ($bytes_transferred > 0 && $filesize >= 8192) {
            $bytes_transferred += 8192;
            if (!isset($filesize)) {
                printf("\rUnknown filesize.. %2d kb done..", $bytes_transferred/1024);
            } else {
                $length = (int)(($bytes_transferred/$filesize)*100);
                printf("\r[%-100s] %d%% (%2d/%2d kb)", str_repeat("=", $length). ">", $length, ($bytes_transferred/1024), $filesize/1024);
            }
        }
        break;
    }
}
?>

A couple of unreported behaviors:
- case 1) if this callback calls die/exit($msg), it will print $msg, then the execution will go on until the request/wrapper is consumed, emitting a "PHP Warning: Failed to call user notifier" on each invocation of the callback. After the last callback invocation, the script is immediately terminated.
- case 2) if this callback throws an exception, it will behave the same way as exit/die, except for the fact that after the last callback invocation it does not terminate the script. The exception is instead raised in the scope of the request/wrapper, and can be caught with a try catch (right there or at a higher level).

Example code for case 1). The final "TEST ECHO" string will NOT be printed.
<?php
$context = stream_context_create(['http' => ['ignore_errors' => true,]]);
stream_context_set_params($context, ['notification' => function () {
    die('error');
}]);

file_get_contents('https://www.google.com', false, $context);
echo "TEST ECHO";
?>

Example code for case 2). The exception is raised at the level of the file_get_contents call, it's catched, and the final "TEST ECHO" gets printed.
<?php
$context = stream_context_create(['http' => ['ignore_errors' => true,]]);
stream_context_set_params($context, ['notification' => function () {
    throw new Exception('...');
}]);

try{
    file_get_contents('https://www.google.com', false, $context);
}catch(exception $e) { }
echo "TEST ECHO";
?>