The bounding box is calculated using information available from character metrics, and unfortunately tends to differ slightly from the results achieved by actually rasterizing the text. If the angle is 0 degrees, you can expect the text to need 1 pixel more to every direction.

Beginning with PHP 4.3, bits and channels are present for other image types, too. However, the presence of these values can be a bit confusing. As an example, GIF always uses 3 channels per pixel, but the number of bits per pixel cannot be calculated for an animated GIF with a global color table.