苦苦挣扎（理解）PHP异步请求

I'm really struggling to understand the way PHP asynchronous requests work. It seems to me that there cannot be any true asynchronous behaviour in PHP because to receive a response from an asynchronous request, you have to delay execution of the script (blocking, essentially) which then defeats the purpose.. right?

What I'm trying to do from a page on my site is:

1. Initiate AJAX request to (local) controller to begin the remote API requests
2. Use a loop of GuzzleHTTP requests to remote API
3. Upon completion of each request, send the result to a socket.io server which then emits a message back to the listening client (the original page that initiated the AJAX request)

Now, I have this working - with one caveat: the GuzzleHTTP requests are not asynchronous and the remote API is very slow (usually takes around 10 seconds to receive the JSON response) so when there are 10 tasks, my connection to the server is frozen for over a minute. Opening new tabs/windows and trying to access the site results in waiting until the original script run has completed.

This seems to indicate that I misunderstood how HTTP requests work. I thought that each individual browser window (or request) was completely separate as far as the server is concerned but from the result I'm seeing, perhaps this is not exactly the case?

This is why I started looking into asynchronous requests and discovered that they don't really seem all that.. asynchronous.

So what I'm really looking for is help filling the gaps/misunderstandings in my knowledge of:

• Why the blocking occurs in the first place (in terms of new, completely separate requests)
• Where I might look to implement a solution.