Prepend a dropdown in SearchWP Shortcode search forms with Supplemental Engines

searchwp-customizations.php

<?php
/*
Plugin Name: SearchWP Customizations
Plugin URI: 
Description: 
Version: 
Author: Jonathan Christopher
Author URI: 
License: 
License URI: 
*/

function my_searchwp_shortcodes_before_input() {
	$settings = SWP()->settings;
	$engines_settings = isset( $settings['engines'] ) ? $settings['engines'] : array();

	if ( empty( $engines_settings ) ) {
		return;
	}

	// get all supplemental engine names
	$engines = array();
	foreach ( $engines_settings as $engine_name => $engine_settings ) {
		if ( isset( $engine_settings['searchwp_engine_label'] ) ) {
			$engines[ $engine_name ] = $engine_settings['searchwp_engine_label'];
		}
	}

	if ( empty( $engines ) ) {
		return;
	}

	$active_engine = isset( $_GET['my_searchwp_engine'] ) ? $_GET['my_searchwp_engine'] : false;
	?>

	<select name="my_searchwp_engine">
		<?php foreach ( $engines as $engine_name => $engine_label ) : ?>
			<option value="<?php echo esc_attr( $engine_name ); ?>" <?php selected( $active_engine, $engine_name ); ?>><?php echo esc_html( $engine_label ); ?></option>
		<?php endforeach; ?>
	</select>

<?php }

add_action( 'searchwp_shortcodes_before_input', 'my_searchwp_shortcodes_before_input' );

// dynamically swap out the engine config when applicable
function my_searchwp_shortcodes_engine( $engine ) {

	if ( ! isset( $_GET['my_searchwp_engine'] ) ) {
		return $engine;
	}

	$engine_name = sanitize_key( $_GET['my_searchwp_engine'] );
	$settings = SWP()->settings;

	// if valid engine name, return that
	return isset( $settings['engines'][ $engine_name ] ) ? $engine_name : $engine;
}

add_filter( 'searchwp_shortcodes_engine', 'my_searchwp_shortcodes_engine' );

function my_searchwp_shortcodes_pagination_engine_fix( $url ) {
	if ( isset( $_GET['my_searchwp_engine'] ) ) {
		$url = remove_query_arg( 'engine', $url );
		$url = add_query_arg( array(
				'my_searchwp_engine' => sanitize_key( $_GET['my_searchwp_engine'] ),
			), $url );
	}

	return esc_url( $url );
}

add_filter( 'searchwp_shortcodes_pagination_prev', 'my_searchwp_shortcodes_pagination_engine_fix' );
add_filter( 'searchwp_shortcodes_pagination_next', 'my_searchwp_shortcodes_pagination_engine_fix' );