Files were randomly assigned into n small files.

rand_cut_file.php

<?php

$fileName = __DIR__ . '/file.txt';
$fp = fopen($fileName, 'r');
while(!feof($fp)) {
    $buffer = fgets($fp, 4096);
    $k = mt_rand(0, 9);
    $res[$k][] = $buffer;
}
fclose($fp);

foreach ($res as $k => $v) {
    shuffle($v);
    $fileStr = implode("", $v);
    file_put_contents(__DIR__ . "/file_$k.txt", $fileStr);
}