How to base64-decode large files in PHP

Question

My PHP web application has an API that can recieve reasonably large files (up to 32 MB) which are base64 encoded. The goal is to write these files somewhere on my filesystem. Decoded of course. What would be the least resource intensive way of doing this?

Edit: Recieving the files through an API means that I have a 32MB string in my PHP app, not a 32 MB source file somewhere on disk. I need to get that string decoded an onto the filesystem.

Using PHP's own base64_decode() isn't cutting it because it uses a lot of memory so I keep running into PHP's memory limit (I know, I could raise that limit but I don't feel good about allowing PHP to use 256MB or so per process).

Any other options? Could I do it manually? Or write the file to disk encoded and call some external command? Any thought?

Answer 1

Even though this has an accepted answer, I have a different suggestion.

If you are pulling the data from an API, you should not store the entire payload in a variable. Using curl or other HTTP fetchers you can automatically store your data in a file.

Assuming you are fetching the data through a simple GET url:

$url = 'http://www.example.com/myfile.base64';
$target = 'localfile.data';

$rhandle = fopen($url,'r');
stream_filter_append($rhandle, 'convert.base64-decode');

$whandle = fopen($target,'w');

stream_copy_to_stream($rhandle,$whandle);
fclose($rhandle);
fclose($whandle);

Benefits:

• Should be faster (less copying of huge variables)
• Very little memory overhead

If you must grab the data from a temporary variable, I can suggest this approach:

$data = 'your base64 data';
$target = 'localfile.data';

$whandle = fopen($target,'w');
stream_filter_append($whandle, 'convert.base64-decode',STREAM_FILTER_WRITE);

fwrite($whandle,$data);

fclose($whandle);

Answer 2

Decode the data in smaller chunks. Four characters of Base64 data equal three bytes of “Base256” data.

So you could group each 1024 characters and decode them to 768 octets of binary data:

$chunkSize = 1024;
$src = fopen('base64.data', 'rb');
$dst = fopen('binary.data', 'wb');
while (!feof($src)) {
    fwrite($dst, base64_decode(fread($src, $chunkSize)));
}
fclose($dst);
fclose($src);