Passando valores por $_POST e pegando dentro do post.php !!!

<script>
jQuery(document).ready(function(){
    jQuery('#setting-add-fanfic').submit(function(){

        var fanfic_id = $("input[name='fanfic_id']").val();
        var user_id = $("input[name='user_id']").val();

        jQuery.ajax({
            type: "POST",
            url: "<?php echo base_url("functions_extra/post.php"); ?>",
            data: {fanfic_id: fanfic_id, user_id: user_id},
            success: function( data ){
                $('#alert').html('<?php echo $wo['lang']['success'];?>');
            }
        });
        return false;
    });
});
</script>

// Arquivo post.php

$fanfic_id = $_POST["fanfic_id"];
$user_id = $_POST["user_id"];