PHP: How to solve ob_start() in combination imagepng() issue?

Question

I use the following code to create an image and encode it to base64. There is no direct output of the image.

ob_start(); // catching the output buffer
imagepng($imgSignature);
$base64Signature=base64_encode(ob_get_contents());
ob_end_clean();

ob_start started recently to throw error 500 and I have trouble figuring out the issue. The server uses php 5.4.11. I really don't know if it was running the same version as I installed the script, of if the memory runs full. I know that ob_start has changed throughout the php version. I really have a hard time to wrap my head around this. Is the script correct for php 5.4.11?

I really appreciate any help.

Answer 1

I'm not sure how to solve your issue with ob_start(), but I have an alternative for what you are doing that don't envolve output buffers.

imagepng($imgSignature, 'php://memory/file.png');
$base64Signature = base64_encode(file_get_contents('php://memory/file.png'));

This is basically saving the png image to a virtual temporary file that exists only in memory, then you read it back and have the same result.

My theory about your error:

At some point in your code, you will have this image stored multiple times in memory. In the $imgSignature, the internal buffer you created with ob_start(), the buffer you read with ob_get_contents(), and the resulting value of base64_encode(). Pretty much all in one line. God only knows how much memory its using, not to mention you probably allocated more resources before as you were mounting this image.

It is important to not have too much stuff allocated at the same time, specially when dealing with memory consuming resources like images. If you unset() or overwrite variables you no longer need, you will allow the garbage collector to do its job of disposing those unreferenced resources from memory.

For instance, you can change the way this piece of code was written to this:

ob_start();
imagepng($imgSignature);
imagedestroy($imgSignature);
$data = ob_get_contents();
ob_end_clean();
$data = base64_encode($data);

I dropped $imgSignature as soon as I didn't need it anymore, ended and cleaned my buffer as soon I was done getting what I wanted from it, and then disposed $data as I overwrote it with the base64 encoded $data that was really what I wanted.

Now this will use significantly less memory. If you extend this to the rest of your code, or do it at least to the parts that use a lot of memory like the images you loaded or created with the GD2 lib, it should optimize the memory usage of your script giving you that extra space you need.