Description:
------------
In a script, we are using PHP and GD function imagecreatefromstring to read image files and check if they are valid images. We don't know at this point if it is a jpeg/png/..., it could be a base64 encoded string too. This gave us a return value of true if the image was valid, and false if the image was invalid, which was grat.

Now, the same code PHP gives a Fatal Error, and we can't find a way to implement the same check as before...

We don't really care about the warnings GD emits: we know some of the files will be "invalid". But we really care about the bool(false) result as this was our way to check the file.

NB: invalid images could be Apple crushed PNGs like in the test script file.


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

$b = file_get_contents('https://www.gregoriol.net/tmp/AppIcon.png');

$r = imagecreatefromstring($b);

var_dump($r);


Expected result:
----------------
$ php -v
PHP 7.0.8-0ubuntu0.16.04.3 (cli) ( NTS )
Copyright (c) 1997-2016 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2016 Zend Technologies
    with Zend OPcache v7.0.8-0ubuntu0.16.04.3, Copyright (c) 1999-2016, by Zend Technologies

$ php test-imagecreatefromstring.php
PHP Unknown error:  imagecreatefromstring(): gd-png: fatal libpng error: CgBI: unknown critical chunk
 in Unknown on line 0
PHP Parse error:  imagecreatefromstring(): gd-png error: setjmp returns error condition 1
 in test-imagecreatefromstring.php on line 5
PHP Warning:  imagecreatefromstring(): Passed data is not in 'PNG' format in test-imagecreatefromstring.php on line 5
PHP Warning:  imagecreatefromstring(): Couldn't create GD Image Stream out of Data in test-imagecreatefromstring.php on line 5
bool(false)


Actual result:
--------------
$ php -v
PHP 7.0.13-0ubuntu0.16.04.1 (cli) ( NTS )
Copyright (c) 1997-2016 The PHP Group
Zend Engine v3.0.0, Copyright (c) 1998-2016 Zend Technologies
    with Zend OPcache v7.0.13-0ubuntu0.16.04.1, Copyright (c) 1999-2016, by Zend Technologies

$ php test-imagecreatefromstring.php
PHP Fatal error:  imagecreatefromstring(): gd-png: fatal libpng error: CgBI: unknown critical chunk
 in test-imagecreatefromstring.php on line 5