Description:
------------
In file https://github.com/php/php-src/blob/master/ext/gd/libgd/gd_gif_out.c#L336, line 336:
        B |= (Resolution - 1) << 5;
Should be:
        B |= (Resolution - 1) << 4;
This bug was fixed in the original libgd: 
https://github.com/libgd/libgd/commit/db10f301fff50d8f65ebbbedb57b79433c2dc8a3
But still presents in PHP version of GD

Test script:
---------------
<?php
$im = imagecreate(3, 3); // 3x3, 9 colors, 4 bits per pixel
for ($x = 0; $x < 3; $x++)
	for ($y = 0; $y < 3; $y++)
		imagesetpixel($im, $x, $y, imagecolorallocate($im, $x, $y, 0));
ob_start();
imagegif($im);
echo decbin(ord(ob_get_clean()[0xA]));

Expected result:
----------------
Should output "10110011" (1 011 0 011, as Resolution bits and BitsPerPixel bits should match)

Actual result:
--------------
Outputs "11100011" (1 110 0 011, Resolutions bits are shifted 1 bit left)