为什么会话在页面重新加载时被删除？ - PHP

This is my code:

INDEX.PHP

    // index.php

    session_start();

    $_SESSION['favcolor'] = 'red';
    $_SESSION['animal']   = 'dog';
    $_SESSION['time']     = time();

    print_r($_SESSION);

The result is:

    Array
    (
        [favcolor] => red
        [animal] => dog
        [time] => 1439307117
    )

After visiting the home, I go to the test page:

TEST.PHP

    // test.php

    session_start();

    echo $_SESSION['favcolor'];
    echo $_SESSION['animal'];
    echo date('Y m d H:i:s', $_SESSION['time']);

    print_r($_SESSION);

The result is:

    Array ()

Why?

I solved the problem. I had the folder "var/lib/php/session" without write permissions.

Setting permissions with the command "chmod -R 777 /var/lib/php/session" everything is back to work.

Thanks to all for the tips.