Description:
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The imageinterlace function does not set the interlace bit on GIF images.  It only works on PNG and JPG images.

Reproduce code:
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$fileName = '/path/to/gif/file';
$resImage = @imagecreatefromgif($fileName);
imageinterlace($resImage, 1);
imagegif($resImage, $fileName);
imagedestroy($resImage);

Expected result:
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The interlace bit should be set on the GIF image.

Actual result:
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The interlace bit is not set on the GIF image.