PHP Manual: socket

Description

int socket_select ( array &read, array &write, array &except, int tv_sec [, int tv_usec] )

socket_select() accepts arrays of sockets and waits for them to change status. Those coming with BSD sockets background will recognize that those socket resource arrays are in fact the so-called file descriptor sets. Three independent arrays of socket resources are watched.

The sockets listed in the read array will be watched to see if characters become available for reading (more precisely, to see if a read will not block - in particular, a socket resource is also ready on end-of-file, in which case a socket_read() will return a zero length string).

The sockets listed in the write array will be watched to see if a write will not block.

The sockets listed in the except array will be watched for exceptions.

Warning

On exit, the arrays are modified to indicate which socket resource actually changed status.

You do not need to pass every array to socket_select(). You can leave it out and use an empty array or NULL instead. Also do not forget that those arrays are passed by reference and will be modified after socket_select() returns.

Example 1. socket_select() example

<?php
/* Prepare the read array */
$read = array($socket1, $socket2);

$num_changed_sockets = socket_select($read, $write = NULL, $except = NULL, 0);

if ($num_changed_sockets === false) {
    /* Error handling */
} else if ($num_changed_sockets > 0) {
    /* At least at one of the sockets something interesting happened */
}
?>

Note: Due a limitation in the current Zend Engine it is not possible to pass a constant modifier like NULL directly as a parameter to a function which expects this parameter to be passed by reference. Instead use a temporary variable or an expression with the leftmost member being a temporary variable:
Example 2. Using NULL with socket_select()
<?php
socket_select($r, $w, $e = NULL, 0);
?>

The tv_sec and tv_usec together form the timeout parameter. The timeout is an upper bound on the amount of time elapsed before socket_select() return. tv_sec may be zero , causing socket_select() to return immediately. This is useful for polling. If tv_sec is NULL (no timeout), socket_select() can block indefinitely.

On success socket_select() returns the number of socket resources contained in the modified arrays, which may be zero if the timeout expires before anything interesting happens. On error FALSE is returned. The error code can be retrieved with socket_last_error().

Note: Be sure to use the === operator when checking for an error. Since the socket_select() may return 0 the comparison with == would evaluate to TRUE:
Example 3. Understanding socket_select()'s result
<?php
if (false === socket_select($r, $w, $e = NULL, 0)) {
    echo "socket_select() failed, reason: " .
        socket_strerror(socket_last_error()) . "\n";
}
?>

Note: Be aware that some socket implementations need to be handled very carefully. A few basic rules:
You should always try to use socket_select() without timeout. Your program should have nothing to do if there is no data available. Code that depends on timeouts is not usually portable and difficult to debug.
No socket resource must be added to any set if you do not intend to check its result after the socket_select() call, and respond appropriately. After socket_select() returns, all socket resources in all arrays must be checked. Any socket resource that is available for writing must be written to, and any socket resource available for reading must be read from.
If you read/write to a socket returns in the arrays be aware that they do not necessarily read/write the full amount of data you have requested. Be prepared to even only be able to read/write a single byte.
It's common to most socket implementations that the only exception caught with the except array is out-of-bound data received on a socket.