Can PHP asynchronously use sockets?

Yup, that's what socket_set_nonblock() is for. Your socket interaction code will need to be written differently, taking into account the special meanings that error codes 11, EWOULDBLOCK, and 115, EINPROGRESS, assume.

Here's some somewhat-fictionalized sample code from a PHP sync socket polling loop, as requested:

$buf = '';$done = false;do {    $chunk = socket_read($sock, 4096);    if($chunk === false) {        $error = socket_last_error($sock);        if($error != 11 && $error != 115) {            my_error_handler(socket_strerror($error), $error);            $done = true;        }        break;    } elseif($chunk == '') {        $done = true;        break;    } else {         $buf .= $chunk;    }} while(true);

How do I do the same asynchronously? so I can respond to data in a data received event, instead of polling for data, etc.

You will need to execute your script and issue stream_select to check weither there is any data to receive. Process and send data back.

The term "asynchronous" is often misused in network programming. For I/O, asynchronous is often just used as another word for non-blocking. This means that the process is able to continue before a call on the network api has completed transmission.

For process execution in general, asynchronous means that multiple instructions are able to be computed at once (concurrently.)

In other words, asynchronous I/O is not truly asynchronous unless multiple threads are used to allow multiple reads/write/accepts to occur concurrently - all sockets will sill have to wait on a synchronous non-blocking call if it has data to be read/written or will otherwise not block, and reading/writing a large file can still take seconds or even minutes if not interrupted. Note that this would require a perfect flow between the client and server or TCP itself will interrupt the transmission. For example, a server sending faster than a client can download would cause a block on a write.

So from a strict point of view PHP is not able to perform asynchronous networking, only non-blocking. In short, the progression of the process will stop while the network call is able to usefully read/write etc. However, the process will then continue when the call is not able to usefully read/write or would otherwise block. In a truly asynchronous system the process will continue regardless, and the read/write will be done in a different thread. Note that blocking I/O can still be done asynchronously if done in a different thread.

Moreover, PHP is not able to do event driven I/O without installing an extension that supports it. You will otherwise need to do some form of polling in order to do non-blocking I/O in PHP. The code from Chaos would be a functional non-blocking read example if it used socket_select.

With that said, the select function will still allow true non-blocking behavior in PHP. In C, polling services have a performance loss over event driven, so I'm sure that it would be the same for PHP. But this loss is in the nanoseconds-microseconds depending on the amount of sockets, where the time saved from a non-blocking call is typically milliseconds, or even seconds if the call is made to wait.