Here we have the same function to write a socket but with improved performance.

If the messager are not larger, they will be written entirely with a single socket_write() call. And is not needed to call the substr() function for the first bucle.

<?php
$st="Message to sent";
$length = strlen($st);
        
    while (true) {
        
        $sent = socket_write($socket, $st, $length);
            
        if ($sent === false) {
        
            break;
        }
            
        // Check if the entire message has been sented
        if ($sent < $length) {
                
            // If not sent the entire message.
            // Get the part of the message that has not yet been sented as message
            $st = substr($st, $sent);
                
            // Get the length of the not sented part
            $length -= $sent;

        } else {
            
            break;
        }
            
    }
?>

sending a few mbs or more results in incomplete transfers, send data in a loop and chunks instead, socket_write reports complete write even though it is only a partial transfer, possibly because of buffer overrun somewhere.

$strlen=strlen($msg);
$totaltransferred=0;

$blocksize=10000;
for ($a=0;$a<$strlen;$a+=$blocksize){
    $part=substr($msg,$a,$blocksize);
    $transferred=socket_write($socket,$part,strlen($part));
    $totaltransferred+=$transferred;
}
    
if ($totaltransferred<$strlen){
    echo "incomplete transfer";
}

from http://www.manualy.sk/sock-faq/unix-socket-faq-2.html
read() is equivalent to recv() with a flags parameter of 0. Other values for the flags parameter change the behaviour of recv(). Similarly, write() is equivalent to send() with flags == 0.

I often read in php docs users not checking for the php function returned value, and in the case of socket_write, I could not see here in the comment anyone botering to read on the socket the server reply.
Then one user thought it would be a good idea to use usleep after a socket_write on a smtp connection.
Actually, if you check the server reply, not only will it give time for the server to reply before you write again on the socket, but also this is a great opportunity to check what the server replied you.
For instance, for smtp connection :
In this example MAIL_SERVER, MAIL_PORT and DEBUG are constants I defined.
<?php
function sendmail( $param )
{
    $from    = &$param[ 'from' ];
    $to      = &$param[ 'to' ];
    $message = &$param[ 'data' ];
    
    $isError = function( $string )
    {
        if( preg_match( '/^((\d)(\d{2}))/', $string, $matches ) )
        {
            if( $matches[ 2 ] == 4 || $matches[ 2 ] == 5 ) return( $matches[ 1 ] );
        }
        else
        {
            return( false );
        }
    };
    
    try
    {
        $socket = null;
        if( ( $socket = socket_create( AF_INET, SOCK_STREAM, SOL_TCP ) ) == false )
        {
            throw new Exception( sprintf( "Unable to create a socket: %s", socket_strerror( socket_last_error() ) ) );
        }
        if( !socket_connect( $socket, MAIL_SERVER, MAIL_PORT ) ) 
        {
            throw new Exception( sprintf( "Unable to connect to server %s: %s", MAIL_SERVER, socket_strerror( socket_last_error() ) ) );
        }
        $read = socket_read( $socket, 1024 );
        if( $read == false )
        {
            throw new Exception( sprintf( "Unable to read from socket: %s", socket_strerror( socket_last_error() ) ) );
        }
        
        if( socket_write( $socket, sprintf( "HELO %s\r\n", gethostname() ) ) === false )
        {
            throw new Exception( sprintf( "Unable to write to socket: %s", socket_strerror( socket_last_error() ) ) );
        }
        $read = socket_read( $socket, 1024 );
        if( $read == false )
        {
            throw new Exception( sprintf( "Unable to read from socket: %s", socket_strerror( socket_last_error() ) ) );
        }
        else
        {
            if( ( $errCode = $isError( $read ) ) ) throw new Exception( "Server responded with an error code $errCode" );
        }
        
        if( socket_write( $socket, sprintf( "MAIL FROM: %s\r\n", $from ) ) === false )
        {
            throw new Exception( sprintf( "Unable to write to socket: %s", socket_strerror( socket_last_error() ) ) );
        }
        $read = socket_read( $socket, 1024 );
        if( $read == false )
        {
            throw new Exception( sprintf( "Unable to read from socket: %s", socket_strerror( socket_last_error() ) ) );
        }
        else
        {
            if( ( $errCode = $isError( $read ) ) ) throw new Exception( "Server responded with an error code $errCode" );
        }
        /* And some more code, but not enough place in comment */
        return( $totalWriten );
    }
    catch( Exception $e )
    {
        $ERROR = sprintf( "Error sending mail message at line %d. ", $e->getLine() ) . $e->getMessage();
        return( false );
    }
}

If you connect to a Server in a way like you do with telnet or some similar protokoll you may have problems with sending data to the server. I found out that at some servers there is a different between:

<?php
    
    socket_write ($my_socket, $line, strlen ($line));
    socket_write ($my_socket, "\r\n", strlen ("\r\n"));
    
?>
witch worked at least, and 
<?php
    socket_write ($my_socket, $line."\r\n", strlen ($line."\r\n"));
?>
wich made the server stop sending any data.

I hope this helps to save a lot of time. I needed about two days to find out, that this was the problem ;)