(PHP 4 >= 4.0.6, PHP 5)

imagecolorresolvealpha --  Get the index of the specified color + alpha or its closest possible alternative


int imagecolorresolvealpha ( resource image, int red, int green, int blue, int alpha )

This function is guaranteed to return a color index for a requested color, either the exact color or the closest possible alternative.

Note: This function requires GD 2.0.1 or later.

See also imagecolorclosestalpha().