How do I get variables from my plugin’s settings page?

In your options page each field should be put into an options array. For Example:

When you registered your settings page you should have used a code like this:

register_setting( 'pluginPage', 'rt_fed_settings', 'rt_validate');

Once on your page where you want to call settings, use this code:

$options_fed = get_option('rt_fed_settings'); // this is calling your settings array into a variable called options_fed.

Now to call items from it you would call them just like any array items:

$options_fed['rt_fed_text_test_billing_field']

The array items were set in your plugins page, but if you want see what is in the code you can use print_r.