base64_encode（$ image）图像大小压缩 - PHP

I have couple of png transparent layer to merge. All images combine and generate an image. But the image has more than 1 MB in size. I can't push all the code here but my idea is

1. Add multiple PNG transparent layers
2. Convert into base64 encode string and return to the browser
3. Show the image

Here is my PHP FIDDLE to check the code from line no. 77 to 81. You can see my complete code. I need to compress the image size of base64 encoded image. Is there any way to how to do that ? Thanks in advance