How to resize image by GD in PHP without color noise?

[HongThao]

Hi all,

I need to change image's size by GD in PHP. An fix image at the first layer, the second layer has an transparent image with white background. We can't see this white background because I did:
$trans = imagecolortransparent($layer_2,$white);
There are 2 buttons "Zoom in" and "Zoom out" that user can change image size whatever they want. My problem is:
After changing the first time it's fine. Continue to change one more time the image's background has color noise.
Does anyone have idea that help to solve the background color noise?
Thanks so much,
Hong Thao