(PHP 4 >= 4.0.6)
ImageColorClosestAlpha -- Get the index of the closest color to the specified color + alphaReturns the index of the color in the palette of the image which is "closest" to the specified RGB value and alpha level.
See also imagecolorexactalpha().
Note: This function was added in PHP 4.0.6 and requires GD 2.0.1