Confirmed for master snapshot and 5.5.10.

A quick dive into the code suggests `gmImageRotateBilinear` contains code that handles floats inproperly by casting them to `int` (hence truncating) where extracting integer part was probably meant.

Minimal code:

$image = imagerotate(imagecreate(10, 10), -90, 0)
echo imagesy($image), 'x', imagesx($image)

If the source image has size of 1x1, the effects are even worse, as the functions below receive 0 as size.

A typo. I meant `gdImageRotateBilinear` of course, not `gm...`.

Might be a duplicate of bug #65148.

I agree. I do appear to have reported a duplicate of #65148, the reporter observed that rotations by negative degrees truncated the picture.
I will aim to build up to date php5.4, 5.5 and 5.6 this week and test.

The issue is rather clear for square angles. As of PHP 5.5.0 the
rotation is done by gdImageRotateInterpolated()[1], which has
special cases for 90, 180 and 270 degrees, but not for their
negative counterparts. That has already been fixed in the external
libgd by adding special cases for -90, -180 and -270 degress[2].

I'm going to port these changes to PHP's bundled libgd as fix for
this ticket. I'm leaving the other ticket open, because the user
comments seem to refer to non-square angles (at least partially),
and these issues deserve some closer investigation.

[1] <https://github.com/php/php-src/blob/php-5.6.11/ext/gd/libgd/gd_interpolation.c#L2156>
[2] <https://github.com/libgd/libgd/blob/gd-2.1.1/src/gd_interpolation.c#L2219-L2229>