Socket Programming With PHP - POP Goes The Weasel
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Let's try something a little more advanced. How about a TCP client that connects to a POP3 server to retrieve the total number of messages for a user's mailbox?

In order to build such a client, I need to first understand the sequence of commands passed to a POP3 server in order to obtain the message total, and then replicate this sequence in my PHP-based client. The best way to do this is by using a regular telnet client to interact with the server and understand the command sequence - so here goes:

$ telnet mail.host 110
Trying 192.168.0.1...
Connected to 192.168.0.1.
Escape character is '^]'.
+OK POP3 mail.host v5.5 server ready
USER john
+OK User name accepted, password please
PASS doe
+OK Mailbox open, 72 messages
STAT
+OK 72 24595628
QUIT
+OK Sayonara
Connection closed by foreign host.

As you can see from the sample session above, the second element of the string returned by a STAT command

STAT
+OK 72 24595628

holds the total number of messages (72, in this case). All that's needed, therefore, is a script that connects to the POP3 server (usually available on port 110), sends the sequence of commands above, retrieves the output of the STAT command, and extracts the message total from it.

Here's the script to accomplish this:

<?
// mail server settings
$host="192.168.0.99";
$port = 110;
$user = "john";
$pass = "doe";

// open a client connection
$fp = fsockopen ($host, $port, $errno, $errstr);

// if a handle is not returned
if (!$fp)
{
	die("Error: could not open socket connection\n");
}
else
{
	// get the welcome message
  	$welcome = fgets ($fp, 150);

	// check for success code
	if (substr($welcome, 0, 3) == "+OK")
	{
	  	// send username and read response
  fputs ($fp, "USER $user\n");
  fgets($fp, 50);

	  	// send password and read response
	  	fputs ($fp, "PASS $pass\n");
  $ack = fgets($fp, 50);

  // check for success code
  if (substr($ack, 0, 3) == "+OK")
  {
    	// send status request and read response
    	fputs ($fp, "STAT\n");
  	$status = fgets($fp, 50);
  	if (substr($status, 0, 3) == "+OK")
  	{
  	  	// shut down connection
    fputs ($fp, "QUIT\n");
  	  	fclose ($fp);
  	}
  	// error getting status
  	else
  	{
    die ("Server said: $status");
  	}
  }
  // auth failure
  else
  {
  	die ("Server said: $ack");
  }
	}
	// bad welcome message
	else
	{
  die ("Bad connection string\n");
	}

	// get status string
	// split by spaces
	$arr = explode(" ", $status);

	// the second element contains the total number of messages
	echo $arr[1] . " messages in mailbox";
}
?>

And here's the output:

$ /usr/local/bin/php -q popclient.php
72 messages in mailbox

How does this work? Very simple.

First, a connection is opened to the POP3 server using the fsockopen() function discussed previously; the arguments to this function (host, port et al) are obtained through PHP variables which are set at the top of the script.

<?
// open a client connection
$fp = fsockopen ($host, $port, $errno, $errstr);
?>

Once a socket connection has been established, the fgets() and fputs() functions are used to send POP3 commands to the server via this socket, and read the resulting output into PHP variables.

<?
	  	// send username and read response
  fputs ($fp, "USER $user\n");
  fgets($fp, 50);

	  	// send password and read response
	  	fputs ($fp, "PASS $pass\n");
  $ack = fgets($fp, 50);
?>

A POP3 server typically prefixes the result of every successful command with the string "+OK". This knowledge allows for simple error-checking within the script - note how the output of fgets() is checked at every stage, with subsequent commands executed only if the previous one was successful.

<?
    	// send status request and read response
    	fputs ($fp, "STAT\n");
  	$status = fgets($fp, 50);
  	if (substr($status, 0, 3) == "+OK")
  	{
  	  	// shut down connection
    fputs ($fp, "QUIT\n");
  	  	fclose ($fp);
  	}
  	// error getting status
  	else
  	{
    die ("Server said: $status");
  	}
?>

Once the output of the STAT command has been received, the socket is closed, and the result string is split into its constituent parts with PHP's very cool explode() function. The message total is then extracted and printed.

<?
	// get status string
	// split by spaces
	$arr = explode(" ", $status);

	// the second element contains the total number of messages
	echo $arr[1] . " messages in mailbox";
?>

Simple, huh?

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