searchform.php

<form role="search" method="get" class="search-form" action="<?php echo esc_url( home_url( '/' ) ); ?>">
    <label>
        <span class="screen-reader-text">Search for:</span>
        <input type="search" class="search-field" placeholder="Search..." value="<?php echo esc_attr( get_search_query() ); ?>" name="s" title="Search for:" />
    </label>
    <?php
        $engines = \SearchWP\Settings::get_engines();
        $current_engine = isset( $_GET['swpengine'] ) ? esc_attr( $_GET['swpengine'] ) : 'default';
    ?>
    <select name="swpengine" id="swpengine">
        <?php foreach ( $engines as $engine_name => $engine ) : ?>
            <option value="<?php echo esc_attr( $engine_name ); ?>"
                <?php selected( $current_engine, $engine_name ); ?>>
                <?php echo esc_html( $engine->get_label() ); ?>
            </option>
        <?php endforeach; ?>
    </select>
    <input type="submit" class="search-submit" value="Search" />
</form>