Description:
------------
imagepng() and other output functions don't properly report
failure. For instance, if a read-only stream is passed to
imagepng(), the function returns TRUE without any warning or
notice, but obviously the image has not been written to the file.



Test script:
---------------
<?php

$filename = __DIR__ . DIRECTORY_SEPARATOR . 'bug.png';
touch($filename);
$stream = fopen($filename, 'r');
$im = imagecreatetruecolor(100, 100);
var_dump(imagepng($im, $stream));
var_dump(filesize($filename));


Expected result:
----------------
bool(false)
int(0)

Actual result:
--------------
bool(true)
int(0)