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if (!session_id()) session_start();

$file = file('file.txt');
$key = []; $val = [];
 
foreach ($file as $i => $str) {
    if (!($i % 2)) {
        $key[] = $str;
    } else {
        $val[] = $str;
    }
}
 
if (!isset($_SESSION['arr'])) $_SESSION['arr'] = array_combine($key, $val);
 
for ($i = 0; $i < 3; $i++) {
    if (count($_SESSION['arr']) == 0) break;
    $key = array_rand($_SESSION['arr']);
    file_put_contents('file_new.txt', $key . $_SESSION['arr'][$key], FILE_APPEND);
    unset($_SESSION['arr'][$key]);
}