creates a ZIP file and return it as a stream:

create_zip_stream.php

function create_zip_stream($file_paths)
{
    // Create a new ZipArchive object
    $zip = new ZipArchive();

    // Open a memory stream for the zip file
    $zip_stream = fopen('php://memory', 'w+');
    $zip->open($zip_stream, ZipArchive::CREATE);

    // Add the files to the zip
    foreach ($file_paths as $file_path) {
        $zip->addFile($file_path);
    }

    // Close the zip and return the stream
    $zip->close();
    rewind($zip_stream);
    return $zip_stream;
}

$file_paths = ['/path/to/file1.txt', '/path/to/file2.txt'];

// Create the zip stream
$zip_stream = create_zip_stream($file_paths);

// Set the HTTP headers
header('Content-Type: application/zip');
header('Content-Disposition: attachment; filename="files.zip"');

// Output the zip stream
fpassthru($zip_stream);