socket TCP/IP for HTTP communcation - sockets

Question

The following code is example php code for socket tutorial, and I run successfully to collect remote site or localhost webpage.Either the protocol number I used for socket_create is 0 or 6, both
number is also working on that code, Why ? I thought network programming will needs to include TCP and IP for today's window computer together to make the communication possible. Why just need TCP or IP protocol num could make the program code working that doesn't include both protocol num ?
TCP is protocol for transport layer and IP is protocol for network layer for both OSI or classical TCP/IP model
<?php
$protocol = 'tcp';
$get_prot = getprotobyname($protocol);
echo $get_prot."----protocol\n";
if(!($sock = socket_create(AF_INET, SOCK_STREAM, 6)))
{
$errorcode = socket_last_error();
$errormsg = socket_strerror($errorcode);
die("Couldn't create socket: [$errorcode] $errormsg \n");
}
echo "Socket created \n";
if(!socket_connect($sock , '127.0.0.1' , 80))
{
$errorcode = socket_last_error();
$errormsg = socket_strerror($errorcode);
die("Could not connect: [$errorcode] $errormsg \n");
}
echo "Connection established \n";
$message = "GET / HTTP/1.1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8 \r\n\r\n";
$message = "GET / HTTP/1.1\r\n";
$message .= "Host: \r\n";
$message .= "Connection: Close\r\n\r\n";
//Send the message to the server
if( ! socket_send ( $sock , $message , strlen($message) , 0))
{
$errorcode = socket_last_error();
$errormsg = socket_strerror($errorcode);
die("Could not send data: [$errorcode] $errormsg \n");
}
echo "Message send successfully \n";
//Now receive reply from server
if(socket_recv ( $sock , $buf , 6144 , MSG_WAITALL ) === FALSE)
{
$errorcode = socket_last_error();
$errormsg = socket_strerror($errorcode);
die("Could not receive data: [$errorcode] $errormsg \n");
}
//print the received message
echo $buf;
?>

Answer 1

Passing 0 as the protocol will give you a default protocol. AF_INET says you want
IP protocol suite (IPv4 specifically). For a combination of AF_INET and SOCK_STREAM, tcp is the default. A combination of AF_INET6 and SOCK_DGRAM would give you IPv6 and UDP as default.
TCP has the protocol number 6, which you can pass as well if you want to be explicit.