This script prepares the PHP session.save_path directory for spreading session files on multiple levels. It is a replacement for the infamous mod_files.sh script.

prepare_php_session_folder.php

#!/usr/bin/php
<?php
// See http://www.php.net/manual/en/session.configuration.php#ini.session.save-path

function prepare_dir($base, $hash_chars, $depth, $number) {
    if($depth<0) return $number;

    if(!file_exists($base)) {
        mkdir($base, 0777);
        echo "$base\n";
        $number++;
    }

    for($i=0,$l=strlen($hash_chars);$i<$l;$i++) {
        $dir = $base . DIRECTORY_SEPARATOR . $hash_chars[$i];
        $number = prepare_dir($dir, $hash_chars, $depth - 1, $number);
    }

    return $number;
}

function move_sessions($session_folder, $current, $hash_chars, $depth, $number) {
    $dir = dir($current);
    while(($file = $dir->read()) !== false) {
        if($file === '.' || $file === '..') continue;
        $full_name = $current . DIRECTORY_SEPARATOR . $file;
        if(is_dir($full_name)) {
            $number = move_sessions($session_folder, $full_name, $hash_chars, $depth, $number);
        } else if(preg_match("/^sess_[$hash_chars]+$/", $file)) {
            $new_name = $session_folder;
            for($i=0;$i<$depth;$i++) {
                $new_name .= DIRECTORY_SEPARATOR . $file[5 + $i];
            }
            $new_name .= DIRECTORY_SEPARATOR . $file;

            if($full_name !== $new_name) {
                echo "$full_name > $new_name\n";
                rename($full_name, $new_name);
                $number++;
            }
        }
    }
    $dir->close();
    return $number;
}

function main() {
    $save_path = ini_get('session.save_path');
    if(preg_match('/^(\d)+;(.+)$/', $save_path, $result)) {
        $depth = $result[1];
        $session_folder = $result[2];
        echo "Base session folder : $session_folder\n";
        echo "Session folder depth : $depth\n";

        $hash_bits = ini_get('session.hash_bits_per_character');
        echo "Hash bits : $hash_bits\n";

        $hash_chars = '0123456789abcdef';
        if($hash_bits >= 5) {
            $hash_chars .= 'ghijklmnopqrstuv';
            if($hash_bits >= 6) {
                $hash_chars .= 'wxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-,';
            }
        }

        $number = prepare_dir($session_folder, $hash_chars, $depth, 0);
        echo "Created $number folders.\n";

        echo "Moving sessions in the right place...\n";
        $number = move_sessions($session_folder, $session_folder, $hash_chars, $depth, 0);
        echo "Moved $number session files\n";
    } else {
        echo "Session folder : $save_path\n";
        echo "No depth, no need to do anything\n";
    }
}

main();

Great boilerplate!
I updated a little bit for my needs and php7.2 (hash_bits_per_character is removed, so I just set $hash_bits to 5 for now).
Will move 6m session files and then we'll see if it's needed to be increased.