Sending sockets data with a leading length value

Question

I want to send JSON messages from a PHP script to a C# app over a network connection using PHP Sockets.

Usually, for binary protocols, the first 4 bytes of every message must be an integer which represents the length (how many bytes) of the message.

In C# I prefix every message with an integer that tells the length of the message as follow:

byte[] msgBytes = UTF8Encoding.UTF8.GetBytes("A JSON msg");            
byte[] prefixBytes = BitConverter.GetBytes(msgBytes.Length);
byte[] msgToSend = new byte[prefixBytes.Length + msgBytes.Length];
Buffer.BlockCopy(prefixBytes, 0, msgToSend, 0, prefixBytes.Length);
Buffer.BlockCopy(msgBytes, 0, msgToSend, prefixBytes.Length, msgBytes.Length);

As I understand, in PHP the function socket_send only accept strings. So, how can I do the same prefixing in PHP 5.x?

Update: I posted a follow-up question on how to process such prefixed data when received from a network socket.

Answer 1

In PHP strings are binary.

So you need to encode the integer length value as the binary representation of an unsigned integer as a 4-char (4 Octets; 32 bits) string. See pack:

# choose the right format according to your byte-order needs:

l   signed long (always 32 bit, machine byte order)
L   unsigned long (always 32 bit, machine byte order)
N   unsigned long (always 32 bit, big endian byte order)
V   unsigned long (always 32 bit, little endian byte order)

$string = pack('l', $length);

Answer 2

I guess you could use pack() to convert the number of bytes to a binary string. As you send your data over the network, you probably need to convert using the format "N" (unsigned long, always 32 bit, big endian byte order).

Here's an example:

$s="Hello World";
$length=pack("N",strlen($s));
socket_send($sock,$length.$s,4+strlen($s));