Description
bool 
imagealphablending ( resource image, bool blendmode )
    imagealphablending() allows for two different
    modes of drawing on truecolor images. In blending mode, the
    alpha channel component of the color supplied to all drawing function,
    such as imagesetpixel() determines how much of the
    underlying color should be allowed to shine through.  As a result, gd
    automatically blends the existing color at that point with the drawing color,
    and stores the result in the image.  The resulting pixel is opaque.  In
    non-blending mode, the drawing color is copied literally with its alpha channel
    information, replacing the destination pixel.  Blending mode is not available
    when drawing on palette images.
    If blendmode is TRUE, then blending mode is enabled, otherwise
    disabled.
    Returns TRUE on success or FALSE on failure.
   
Note: This function requires GD 2.0.1 or later.