In PHP, opening a named pipe (FIFO) for both receiving (reading) and writing requires careful handling due to the nature of named pipes as a one-way communication channel at any given moment. While you can open the same named pipe file from different processes for reading and writing, a single fopen() call in PHP will typically open it in either read or write mode, not both simultaneously for bidirectional communication within the same file handle.
To achieve bidirectional communication using named pipes in PHP, you would typically use two separate named pipes: one for communication in each direction.
Example of creating and using two named pipes for bidirectional communication:
Create the named pipes.
Code

   <?php
   $pipe_to_server = '/tmp/my_pipe_to_server';
   $pipe_from_server = '/tmp/my_pipe_from_server';

   // Create named pipes if they don't exist
   if (!file_exists($pipe_to_server)) {
       posix_mkfifo($pipe_to_server, 0666);
   }
   if (!file_exists($pipe_from_server)) {
       posix_mkfifo($pipe_from_server, 0666);
   }
   ?>

   Server-side (e.g., a separate PHP script or process):

Code

   <?php
   // Server-side
   $pipe_to_server = '/tmp/my_pipe_to_server';
   $pipe_from_server = '/tmp/my_pipe_from_server';

   // Open pipe for reading from client
   $read_handle = fopen($pipe_to_server, 'r');
   if (!$read_handle) {
       die("Failed to open pipe for reading.\n");
   }

   // Open pipe for writing to client
   $write_handle = fopen($pipe_from_server, 'w');
   if (!$write_handle) {
       die("Failed to open pipe for writing.\n");
   }

   echo "Server ready.\n";

   // Read from client
   while (!feof($read_handle)) {
       $message = trim(fgets($read_handle));
       if ($message === 'exit') {
           break;
       }
       echo "Server received: " . $message . "\n";
       // Write a response back to the client
       fwrite($write_handle, "Server acknowledges: " . $message . "\n");
   }

   fclose($read_handle);
   fclose($write_handle);
   echo "Server closed.\n";
   ?>

   Client-side (e.g., another PHP script or process):

Code

   <?php
   // Client-side
   $pipe_to_server = '/tmp/my_pipe_to_server';
   $pipe_from_server = '/tmp/my_pipe_from_server';

   // Open pipe for writing to server
   $write_handle = fopen($pipe_to_server, 'w');
   if (!$write_handle) {
       die("Failed to open pipe for writing.\n");
   }

   // Open pipe for reading from server
   $read_handle = fopen($pipe_from_server, 'r');
   if (!$read_handle) {
       die("Failed to open pipe for reading.\n");
   }

   echo "Client ready.\n";

   // Send messages to server and read responses
   fwrite($write_handle, "Hello from client!\n");
   echo "Client received: " . trim(fgets($read_handle)) . "\n";

   fwrite($write_handle, "How are you?\n");
   echo "Client received: " . trim(fgets($read_handle)) . "\n";

   fwrite($write_handle, "exit\n"); // Signal server to exit

   fclose($write_handle);
   fclose($read_handle);
   echo "Client closed.\n";
   ?>

Explanation:

   posix_mkfifo(): This function creates the named pipe files in the filesystem.
   Separate fopen() calls: Each process opens the relevant pipe in either read ('r') or write ('w') mode.
   Dedicated pipes for each direction: pipe_to_server is used for the client to write to the server, and pipe_from_server is used for the server to write back to the client.

This approach effectively enables bidirectional communication between two processes using named pipes in PHP.