如何使用proc_open从仅打开一次的文件中获取多个结果？

I have a command run using exec() which returns a value from within a very large data file, but it has to run millions of times. In order to avoid having the file opened each time in a loop, I want to move to a proc_open-based solution, where the file pointer is created once for efficiency, but can't work out how to do this.

Here is the exec-based version, which works but is slow, presumably because it has to open the file in each iteration:

foreach ($locations as $location) {
    $command = "gdallocationinfo -valonly -wgs84 datafile.img {$location['lon']} {$location['lat']}";
    echo exec ($command);
}

My attempt at a proc_open-based code is as follows:

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

$command = "gdallocationinfo -valonly -wgs84 datafile.img";
$fp = proc_open ($command, $descriptorspec, $pipes);

foreach ($locations as $location) {
    fwrite ($pipes[0], "{$location['lon']} {$location['lat']}
");
    fclose ($pipes[0]);
    echo stream_get_contents ($pipes[1]);
    fclose ($pipes[1]);
}

proc_close ($fp);

This correctly gets the first value, but then generates an error for each subsequent iteration:

3.3904595375061 

Warning: fwrite(): 6 is not a valid stream resource in file.php on line 11
Warning: fclose(): 6 is not a valid stream resource in file.php on line 12
Warning: stream_get_contents(): 7 is not a valid stream resource in file.php on line 13
Warning: fclose(): 7 is not a valid stream resource in file.php on line 14

Warning: fwrite(): 6 is not a valid stream resource in file.php on line 11
...

1. You're not "opening a file" you're executing a process. If that process is not designed to handle multiple requests within the scope of a single execution you're not going to be able to get around that with proc_open() or anything else.

2. In the following block you're closing both the input and output pipes of the process, and yet you're surprised when you're no longer able to read or write?

   foreach ($locations as $location) {
       fwrite ($pipes[0], "{$location['lon']} {$location['lat']}
   ");
       fclose ($pipes[0]); // here
       echo stream_get_contents ($pipes[1]);
       fclose ($pipes[1]); // and here
   }
   

   Try not doing that.