HTML. How to pass check box value to php?

I have form and here is check box, how to pass value from check box to PHP? For example.: If checkbox is checked pass 1, if not checked 0.

I'm using following:

<input type="checkbox" name="letter" value="letter" id="letter" checked="checked">

This is my form:

  <form class="form"  method="post" action="GetFbId.php">
    <h2>Registration</h2><hr/>
    <label>First Name: </label>
    <input type="text" name="first_name" id="first_name" required>

    <label>Last Name: </label>
    <input type="text" name="last_name" id="last_name" required>

    <label>City: </label>
    <input type="text" name="city" id="city" required>

    <label>Email: </label>
    <input type="text" name="email" id="email" required><br>

    Accept to get letters 
    <input type="checkbox" name="letter" value="letter" id="letter" checked="checked">

    <input type="submit" name="register" id="register" value="Register">
  </form>