functions.php

<?php

// Adds a SearchWP Engines dropdown to the search form
function my_searchwp_get_search_form_with_engines_dropdown( $form ) {
	ob_start(); ?>
		<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 = SWP()->settings['engines'];
				$current_engine = isset( $_GET['swpengine'] ) ? esc_attr( $_GET['swpengine'] ) : 'default';
			?>
			<select name="swpengine" id="swpengine">
				<?php foreach ( $engines as $engine => $engine_settings ) : ?>
					<option value="<?php echo esc_attr( $engine ); ?>"
						<?php selected( $current_engine, $engine ); ?>>
						<?php echo isset( $engine_settings['searchwp_engine_label'] ) ? esc_html( $engine_settings['searchwp_engine_label'] ) : 'Default'; ?>
					</option>
				<?php endforeach; ?>
			</select>
			<input type="submit" class="search-submit" value="Search" />
		</form>
	<?php return ob_get_clean();
}

add_filter( 'get_search_form', 'my_searchwp_get_search_form_with_engines_dropdown' );