PHP Manual
Socket Functions
Runs the select() system call on the given arrays of sockets with a specified timeout

socket_select

(PHP 4 >= 4.1.0, PHP 5, PHP 7, PHP 8)

socket_select — Runs the select() system call on the given arrays of sockets with a specified timeout

Description

socket_select(
    ?array &$read,
    ?array &$write,
    ?array &$except,
    ?int $seconds,
    int $microseconds = 0
): int|false

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

Parameters

read: 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 is also ready on end-of-file, in which case a socket_read() will return a zero length string).
write: The sockets listed in the write array will be watched to see if a write will not block.
except: The sockets listed in the except array will be watched for exceptions.
seconds: The seconds and microseconds together form the timeout parameter. The timeout is an upper bound on the amount of time elapsed before socket_select() return. seconds may be zero , causing socket_select() to return immediately. This is useful for polling. If seconds is null (no timeout), socket_select() can block indefinitely.
microseconds

Warning

On exit, the arrays are modified to indicate which socket 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.

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 #1 Using null with socket_select()

<?php $e = NULL; socket_select($r, $w, $e, 0); ?>

Return Values

On success socket_select() returns the number of sockets 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 #2 Understanding socket_select()'s result

<?php $e = NULL; if (false === socket_select($r, $w, $e, 0)) { echo "socket_select() failed, reason: " . socket_strerror(socket_last_error()) . "\n"; } ?>

Examples

Example #3 socket_select() example


<?php
/* Prepare the read array */
$read   = array($socket1, $socket2);
$write  = NULL;
$except = NULL;
$num_changed_sockets = socket_select($read, $write, $except, 0);

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

Notes

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 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 sockets in all arrays must be checked. Any socket that is available for writing must be written to, and any socket 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.