For those wanting to use stream_socket_client() to connect to a local UNIX socket who can't find documentation on how to do it, here's a (rough) example:

<?php

$sock = stream_socket_client('unix:///full/path/to/my/socket.sock', $errno, $errstr);

fwrite($sock, 'SOME COMMAND'."\r\n");

echo fread($sock, 4096)."\n";

fclose($sock);

?>

The remote_socket argument, in its end (well... after the port), can also contain a "/" followed by a unique identifier. This is especially useful if you want to create multiple persistent connections to the same transport://host:port combo.

Example:
<?php
$socket = stream_socket_client('tcp://mysql.example.com:3306/root', $errorno, $errorstr, $timeout, STREAM_CLIENT_CONNECT | STREAM_CLIENT_PERSISTENT);
?>

Note that while (p)fsockopen() follows a similar scheme, it doesn't have this particular feature.

<?php STREAM_CLIENT_ASYNC_CONNECT ?> opens a non-blocking socket (contradictory to the default blocking mode described in the manual page).

You will have to check for its status using <?php stream_select ?> followed by <?php socket_import_stream ?> and <?php socket_get_option($socket, SOL_SOCKET, SO_ERROR) ?>

You wouldn't normally do this, but when you do, you're more or less using glibc multisocket patterns.

Here's a minimal example:

<?php

// open a "server" on port 7238 in a separate terminal (don't open it if you want to see the error path)
// Netcat: nc -l 7238
// Socat: socat TCP-LISTEN:7238,fork,reuseaddr STDIO

$stream = stream_socket_client("tcp://127.0.0.1:7238", $errno, $errstr, flags: STREAM_CLIENT_CONNECT | STREAM_CLIENT_ASYNC_CONNECT);
while (true) {
        $w = [$stream];
        $r = $e = [];
        $changed = stream_select($r, $w, $e, 1, 0);
        // if $changed is false, there's an error that I'm not handling here
        if (in_array($stream, $w)) {
                break;
        }
}
$socket = socket_import_stream($stream);
$so_error = socket_get_option($socket, SOL_SOCKET, SO_ERROR);
if ($so_error === 0) {
        fwrite($stream, "Hello!\n");
} else {
        // you'll have to look up the constant in <errno.h>
        // for example, 111 is ECONNREFUSED
        echo "There's been an error: errno=$so_error\n";
}
?>