Is there any sane way to make a HTTP request asynchronously in PHP without throwing out the response? I.e., something similar to AJAX - the PHP script initiates the request, does it's own thing and later, when the response is received, a callback function/method or another script handles the response.
One approach has crossed my mind - spawning a new php process with another script for each request - the second script does the request, waits for the response and then parses the data and does whatever it should, while the original script goes on spawning new processes. I have doubts, though, about performance in this case - there must be some performance penalty from having to create a new process every time.
Yes, depending on the traffic of your site, spawning a separate PHP process for running a script could be devastating. It would be more efficient to use shell_exec() to start a background process that saves the output to a filename you already know, but even this could be resource intensive.
You could also have a request queue stored in a database. A single, separate background process would pull the job, execute it, and save the output, possibly setting a flag in the DB that your web process could check.
If you're going to use the DB queue approach, use curl_multi* class of functions to send all queued requests at once. This will limit the execution time of each iteration in your background process to the longest request time.