When using exec() in PHP to run an external command, if that command expects further input (e.g., from stdin), the exec() function will wait for that input, causing your PHP script to hang. To address this, you need to provide the expected input to the external command within the exec() call itself.
Here's how to run a PHP script with exec() when the executed script waits for more input: Provide Input via Standard Input Redirection.
You can redirect a string or the content of a file as standard input to the executed script using the < or <<< operators in the shell command.
Code

   <?php
   // Example: A PHP script that expects input
   // my_script.php
   // <?php
   // $input = trim(fgets(STDIN));
   // echo "Received: " . $input . "\n";
   // ? >

   // Providing a string as input
   $input_string = "Hello from PHP exec!";
   $command = "php my_script.php <<< " . escapeshellarg($input_string);
   exec($command, $output, $return_code);
   echo "Output: " . implode("\n", $output) . "\n";
   echo "Return Code: " . $return_code . "\n";

   // Providing content from a file as input
   // Create a temporary file for input
   file_put_contents('input.txt', 'Input from file.');
   $command_file_input = "php my_script.php < input.txt";
   exec($command_file_input, $output_file, $return_code_file);
   echo "Output from file: " . implode("\n", $output_file) . "\n";
   echo "Return Code from file: " . $return_code_file . "\n";
   unlink('input.txt'); // Clean up the temporary file
   ?>

In this example, escapeshellarg() is used to safely escape the input string, preventing shell injection vulnerabilities. Use proc_open() for More Control.
For more complex scenarios where you need finer control over the input, output, and error streams of the child process, proc_open() is a more powerful alternative to exec(). It allows you to create pipes for stdin, stdout, and stderr, enabling two-way communication.
Code

   <?php
   // Example: A PHP script that expects input
   // my_script.php
   // <?php
   // $input = trim(fgets(STDIN));
   // echo "Received: " . $input . "\n";
   // ? >

   $descriptorspec = array(
       0 => array("pipe", "r"),  // stdin is a pipe that the child will read from
       1 => array("pipe", "w"),  // stdout is a pipe that the child will write to
       2 => array("file", "/tmp/error-output.txt", "a") // stderr is a file to write to
   );

   $process = proc_open('php my_script.php', $descriptorspec, $pipes);

   if (is_resource($process)) {
       // Write to stdin
       fwrite($pipes[0], "Hello from proc_open!");
       fclose($pipes[0]);

       // Read from stdout
       $output = stream_get_contents($pipes[1]);
       fclose($pipes[1]);

       // Close the process
       $return_value = proc_close($process);

       echo "Output: " . $output;
       echo "Return Value: " . $return_value . "\n";
   }
   ?>

proc_open() gives you direct handles to the standard input, output, and error streams of the executed process, allowing you to write to its input and read from its output as needed.