Description
int imagecolortransparent ( resource $image [, int $color] )

Sets the transparent color in the given image.

Parameters

image
An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().

color
A color identifier created with imagecolorallocate()


Return Values
The identifier of the new (or current, if none is specified) transparent color is returned.

Notes
Note: Transparency is copied only with imagecopymerge() and true color images, not with imagecopy() or pallete images.

Note: 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.