This is a Custom Wordpress Nav Walker implementation for a custom Bootstrap Menu.

functions.php

<?php
// So-called menu is below and we will implement this custom menu by extending Walker_Nav_Menu class
/*
<ul>
    <li class="active"><a href="index.html">Home</a></li>
    <li class="drop-down"><a href="">About</a>
        <ul>
            <li><a href="about.html">About Us</a></li>
            <li><a href="team.html">Team</a></li>
            <li><a href="testimonials.html">Testimonials</a></li>
            <li class="drop-down"><a href="#">Deep Drop Down</a>
                <ul>
                    <li><a href="#">Deep Drop Down 1</a></li>
                    <li><a href="#">Deep Drop Down 2</a></li>
                    <li><a href="#">Deep Drop Down 3</a></li>
                    <li><a href="#">Deep Drop Down 4</a></li>
                    <li><a href="#">Deep Drop Down 5</a></li>
                </ul>
            </li>
        </ul>
    </li>
    <li><a href="services.html">Services</a></li>
    <li><a href="portfolio.html">Portfolio</a></li>
    <li><a href="pricing.html">Pricing</a></li>
    <li><a href="blog.html">Blog</a></li>
    <li><a href="contact.html">Contact</a></li>
</ul>
*/

// You should call this class below by using wp_nav_menu() function with as an array argument with the key 'walker'
class Custom_Menu_Walker extends Walker_Nav_Menu {

    // We do not care the container element of ul

    // <ul> // Menu without sub menu
    function start_lvl(&$output, $depth=0, $args=array()) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul>\n";
    }

    // <li class="drop-down"><a href="">
    function start_el(&$output, $item, $depth=0, $args=array(), $id=0) {
        $indent = ($depth) ? str_repeat("\t", $depth) : '';

        // li tag
        $class_names = $value = '';

        $classes = empty($item->classes) ? array() : (array) $item->classes;
        $classes[] = ($args->walker->has_children) ? 'drop-down' : '';
        $classes[] = ($item->current || $item->current_item_ancestor) ? 'active' : '';
        $classes[] = 'menu-item-' . $item->ID;
        
        $class_names = join(' ', apply_filters('nav_menu_css_class', array_filter($classes), $item, $args));
        $class_names = ' class="' . esc_attr($class_names) . '" ';

        $id = apply_filters('nav_menu_item_id', 'menu-item-'.$item->ID, $item, $args);
        $id = ($id) ? ' id="' . esc_attr($id) . '"' : '';

        $output .= $indent . '<li' . $id . $value . $class_names . '>';

        // a tag
        $atts = array();
        $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
        $atts['target'] = ! empty( $item->target )     ? $item->target     : '';
        $atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
        $atts['href']   = ! empty( $item->url )        ? $item->url        : '';

        $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );

        $attributes = '';
        foreach ( $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

        $a_output = $args->before;
        $a_output .= '<a' . $attributes . '>';
        $a_output .= $args->link_before . apply_filters('the_title', $item->title, $item->ID) . $args->link_after;
        $a_output .= '</a>';
        $a_output .= $args->after;

        // append <a></a> tag to the <li> tag
        $output .= apply_filters('walker_nav_menu_start_el', $a_output, $item, $depth, $args);
    }

    // </a></li>
    // function end_el() {}

    // </ul>
    //function end_lvl(&$output, $depth=0, $args=null) { }
}