(PHP 4 >= 4.0.6)

ImageColorClosestAlpha --  Get the index of the closest color to the specified color + alpha


int imagecolorclosestalpha (resource im, int red, int green, int blue, int alpha)

Returns 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