These functions are part of PHP's core and do not require loading an extension.
The gamma_correct_tag function adjusts an HTML color from one gamma to another. Video display hardware is given a gamma rating that describes relatively how bright images appear. Identical images appear lighter on Macintosh hardware than on the typical Wintel clone. The W3C has a nice discussion about color spaces that includes information about gamma values: <http://www.w3.org/Graphics/Color/sRGB.html>.
<? //go from Windows gamma to Macintosh gamma $color = gamma_correct_tag("#CC0000", 2.2, 1.571); print("FONT COLOR=\"$color\">"); print("Sample Text"); print("/FONT>"); ?>