(PHP 4, PHP 5)
imagecolortransparent — Define a color as transparent
( resource $image
[, int $color
The identifier of the new (or current, if none is specified)
transparent color is returned.
Transparency is copied only with imagecopymerge() and
true color images, not with imagecopy() or pallete images.
The transparent color is a property of the image, transparency is not a
property of the color. Once you have set a color to be the transparent
color, any regions of the image in that color that were drawn previously
will be transparent.