I have to check if many URLs have redirects. Right now, I'm using a loop and get_headers(). As you can imagine, this is extremely slow. Would it be possible to call get_headers() many times at the same time? (I believe this is called "threading"?)
If it isn't possible, what other options do I have?
You can use process control functions to fork a PHP process and do multiple things at the same time. Recent versions of the cURL extension also support sending multiple requests at the same time (check out the "multi" functions). cURL might be easier if you're not familiar with forking and monitoring processes.