(PHP 3, PHP 4 >= 4.0.0)
imagecolorclosest -- Get the index of the closest color to the specified colorReturns the index of the color in the palette of the image which is "closest" to the specified RGB value.
The "distance" between the desired color and each color in the palette is calculated as if the RGB values represented points in three-dimensional space.
See also imagecolorexact().