PHP glob contents of all files in a directory removing unique BUT create new 500 line files.

Question

The files in the directory are named 1.txt, 2.txt, 3.txt etc.... The snippet below enters that directory, opens all the *,txt files reading them, removes the dupes and creates one file with all the unique contents. (names in this case).

$files = glob($dirname."/*.txt"); //matches all text files
    $lines = array();
    foreach($files as $file)
    {
    $lines = array_merge($lines, file($file, FILE_SKIP_EMPTY_LINES | FILE_IGNORE_NEW_LINES));
    }
    $lines = array_unique($lines);
    file_put_contents($dirname."/allofthem.txt", implode("\n", $lines));
    }

The above works great for me. Thanks to great help here at stackoverflow.

But, I desire to take it one step further.

Instead of one big duplicate free "allofthem.txt" file, how can I modify the above to create files with a maximum of 5oo lines each from the new data?

They need to go into a new directory eg $dirname."/done/".$i.".txt" I have tried counting in the loop but my efforts are not working and ended up being a mile long.

Again, this beginner needs some expert assistance. Thanks in advance.

Afterthought..

I also attempted to push 500 into an array, increment to another array and save that way. No luck.