The return appears to be the size in bytes of the data written to the socket, or -1 on failure (this could be because of non blocking)
stream_socket_sendto — Sends a message to a socket, whether it is connected or not
Sends the specified data through the socket.
The socket to send data to.
The data to be sent.
The value of flags can be any combination of the following:
|STREAM_OOB||Process OOB (out-of-band) data.|
The address specified when the socket stream was created will be used unless an alternate address is specified in address.
If specified, it must be in dotted quad (or [ipv6]) format.
Returns a result code, as an integer.
Example #1 stream_socket_sendto() Example
/* Open a socket to port 1234 on localhost */
$socket = stream_socket_client('tcp://127.0.0.1:1234');
/* Send ordinary data via ordinary channels. */
fwrite($socket, "Normal data transmit.");
/* Send more data out of band. */
stream_socket_sendto($socket, "Out of Band data.", STREAM_OOB);
/* Close it up */