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<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.

add_filter( 'wp_nav_menu', 'sk_search_primary_nav', 10, 2 );
function sk_search_primary_nav( $menu, $args ) {
    if ( 'primary' !== $args->theme_location ) {
        return $menu;
    }

    return $menu . get_search_form( false );
}