Apparently, in your case imagecropauto() returns false. I can't
reproduce this behavior, however (tested with PHP 5.5.20 and
5.6.15 on Windows). If you still experience this problem on recent
PHP versions, please provide the image (img.png) you've used.

BTW: the $threshhold and $color arguments are needed only if
$mode==IMG_CROP_THRESHOLD.

I've found that imagecropauto() returns FALSE, if there is nothing
to crop, e.g.:

    <?php
    $im = imagecreatetruecolor(100, 100);
    $black = imagecolorallocate($im, 0, 0, 0);
    imagefilledrectangle($im, 0, 0, 99, 99, $black);
    $thumb = imagecropauto($im ,IMG_CROP_WHITE);
    var_dump($thumb);

outputs

    bool(false)

I assume that was the case for your img.png, so I'm changing to
documentation bug.

This bug has been fixed in the documentation's XML sources. Since the
online and downloadable versions of the documentation need some time
to get updated, we would like to ask you to be a bit patient.

Thank you for the report, and for helping us make our documentation better.

And special thanks to jordie at jordie dot org for his note, which
was rather helpful for the documentation.