Using CURL to Download a File in PHP

Today at work I came across an interesting problem. I had to make a REST-style call to a backend server, which would do some processing and return a file. The filename and content type of that file is not known to me at the time I make the call, but the file must be downloaded properly directly to the client. And the file could easily be large, meaning that I can't load it completely into memory before sending it to the client (and even if I could, that would be unwise due to the double-time download).

I'd used CURL in PHP before, but was having problems getting the content type. If I typed the REST URL directly into the browser, it would download successfully, but display a useless byte stream instead of the file itself. After discovering the CURLOPT_HEADER option, I came up with the initial solution of making the call twice: the first time I just got the headers (and not the body, using CURLOPT_NOBODY), and then set the headers using header() before making a second call which would get the body. This worked ... except that the backend team informed me that it's possible that the processing could get pretty extensive, and they didn't want it to be doubled every time. Needless to say, that made my solution pretty stupid.

That's when I dug deeper and discovered CURLOPT_HEADERFUNCTION. With this, you register a callback function which will process the headers.

curl_setopt($c, CURLOPT_HEADERFUNCTION, array(&$this, 'applyHeaders'));

The third parameter here is the function that processes the header. If you're using an instance method, you pass an array containing a reference to the current object and the name of the method. In this case, my callback function looks like this:

function applyHeaders($c, $header) {
    header($header);
    return strlen($header);
}

It simply calls the header() function on the file's headers and returns the length of the header (which is necessary for the callback function to work).

This way, you can keep CURLOPT_RETURNTRANSFER false, meaning you don't have to load the file into memory and it's streamed directly to the browser, and the applyHeaders() method set the content type and filename properly, so it gets downloaded just as it's supposed to. Best of all, you only have to make one call to the backend, and the processing is only done once.

Excellent. And the documentation on this isn't the best, so I figured I should post it on here to help out. And to help me remember.