Description:
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imagerotate() doubles the memory usage instead of functions like imageflip(). Its not really a bug because the function returns a new image resource but as image resolutions grow year-by-year this increases the memory requirements, too.

I think the function should not return a new resource. Instead it should be a added a 4th option to overwrite the original image resource:

imagerotate ( resource $image , float $angle , int $bgd_color [, int $ignore_transparent = 0 [, bool $return = true ]] )

Test script:
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<?php

// image
$url = 'https://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/Pluto-01_Stern_03_Pluto_Color_TXT.jpg/1024px-Pluto-01_Stern_03_Pluto_Color_TXT.jpg';
file_put_contents('../cache4/' . basename($url), fopen($url, 'r'));
$filename = '../cache4/' . basename($url);

echo 'Before imagecreate: ' . round(memory_get_usage() / pow(1024, 2)) . ' MB (Max: ' . round(memory_get_peak_usage() / pow(1024, 2)) . ' MB)<br>' . PHP_EOL;

// create image resource
$image = imagecreatefromjpeg($filename);

echo 'After imagecreate: ' . round(memory_get_usage() / pow(1024, 2)) . ' MB (Max: ' . round(memory_get_peak_usage() / pow(1024, 2)) . ' MB)<br>' . PHP_EOL;

// rotate image
$image = imagerotate($image, 180, 0);

echo 'After imagerotate: ' . round(memory_get_usage() / pow(1024, 2)) . ' MB (Max: ' . round(memory_get_peak_usage() / pow(1024, 2)) . ' MB)<br>' . PHP_EOL;

?>

Expected result:
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Before imagecreate: 0 MB (Max: 0 MB)
After imagecreate: 5 MB (Max: 5 MB)
After imagerotate: 5 MB (Max: 5 MB)

(by using "imagerotate($image, 180, 0, 0, false);")

Actual result:
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Before imagecreate: 0 MB (Max: 0 MB)
After imagecreate: 5 MB (Max: 5 MB)
After imagerotate: 5 MB (Max: 10 MB)