Shutting down is a formality two peers can do before closing their connections. It's not required, but it can help assert your I/O procedures, so it's useful.

If writing is shut down, trying to send will result in a pipe error, and the remote peer will read an empty string after receiving all other pending data.

If reading is shut down, trying to receive will return an empty string, and the remote peer will get a pipe error if they try to send.

Writing should be shut down first between two peers. Remaining data should then be read and handled. If anything is sent at this point it should be a "goodbye" (nothing that requires the remote peer to write back to you). Finally, reading can be shut down.

Selection on a shut down channel will always succeed. Similarly, the remote peer will always succeed at selecting the opposite channel.

Sockets should be first shutdown and then closed.
<?php
// Sample: Closing sockets gracefully
socket_shutdown($sock, 2);
socket_close($sock);
?>

That is not a good example of a graceful shutdown. One should close the sending side of the socket and continue to read until the remote end closes its sending connection.

<explain>
In this case, the TCP client is gracefully disconnected from the server

<?php

define('BUF_SIZE',10);

// create for tcp
$sock = socket_create(AF_INET, SOCK_STREAM, getprotobyname('tcp'));
socket_bind($sock, '127.0.0.1',5200);
socket_listen($sock,1024);
$fp = fopen('./socket_shutdown.php','rb');
$clnt_sock = socket_accept($sock);

while(!feof($fp)) {
   $str = fread($fp,BUF_SIZE);
   socket_write($clnt_sock,$str,BUF_SIZE);
} 

$eof = "\n";
socket_write($clnt_sock,$eof,strlen($eof));

//disconnect output stream(断开输入流)
socket_shutdown($clnt_sock,1);
$ret = socket_read($clnt_sock, 100);
printf("Message from client:%s\n",$ret);
socket_close($clnt_sock);
socket_close($sock);
?>

<?php
// for tcp-client 
$clnt_sock = socket_create(AF_INET, SOCK_STREAM, getprotobyname('tcp'));
socket_connect($clnt_sock, '127.0.0.1', 5200);
while (($cnt= @socket_read($clnt_sock, 10,PHP_NORMAL_READ)) !==false) {
   file_put_contents('./receive.data',$cnt,FILE_APPEND);
}
print "receive file data".PHP_EOL;
socket_write($clnt_sock, "Tank you");
socket_close($clnt_sock);
?>

Shutdown and SOL_TCP:
<?php
$a= socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
socket_shutdown($a, 2)
?>
PHP Warning:  socket_shutdown(): unable to shutdown socket [107]: Transport endpoint is not connected 

Shutdown and SOL_UDP:
<?php
$a= socket_create(AF_INET, SOCK_STREAM, SOL_UDP);
socket_shutdown($a, 2)
?>
PHP Warning:  socket_shutdown(): unable to shutdown socket [107]: Transport endpoint is not connected 

Conclusion: if you are not actually connected, shutdown will fails with socket_error = 107, Transport endpoint is not connected. This is true for both TPC and UDP connection (which is suprising, UDP being a connectionless protocol). This is true no matter the value set for the how parameter.