PHP/Graphics Image/Transparent

Making Colors Transparent with imagecolortransparent()

<?php
header("Content-type: image/gif");
$image = imagecreate( 200, 200 );
$red = imagecolorallocate($image, 255,0,0);
$blue = imagecolorallocate($image, 0,0,255 );
$points = array (10, 10,
                 190, 190,
                 190, 10,
                 10, 190
);
imagefilledpolygon( $image, $points, count( $points )/2 , $blue );
imagecolortransparent( $image, $red );
imagegif($image);
?>