Alpha Channel Question

I'm having a small problem understanding Alpha channels. I thought that the Alpha Channel was the level of transparency but the following code does not seem to work. $DI = 50; $MOD = 127/$DI; ImageAlphaBlending($im, true); for ($x=0; $x<$DI; $x++) {

       $AL = 127-$MOD*$x;
       $GLColor=ImageColorResolveAlpha($im,255,255,255,$AL);
       ImageFilledArc($im, $WD/2, $HT/2, $DI-$x, $DI-$x, 0, 360, $GLColor, 4);

} The idea is to create a simple glare effect by taking a white circle and making it more transparent around the edges. Though this only produces a white circle with fuzzy edges. Can anyone tell me what I'm doing wrong and clue me in on this whole Alpha Channel Biz. -Jim _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com