Custom Search using GET params In WordPress

custom_search.php

<?php
/**
 * @fileoverview custom filter and search
 *
 * References
 * @see http://michaelsoriano.com/how-to-create-an-advanced-search-form-for-wordpress/
 * @see http://wordpress.stackexchange.com/a/50051/36130
 * @see https://codex.wordpress.org/Class_Reference/WP_Query#Taxonomy_Parameters
 */

class namespace_search {

	/**
	 * Taxonomy array for tax query
	 */
	private $item = array();

	/**
	 * Initial tax query array
	 */
	private $list = array();

	/**
	 * Form search terms
	 */
	public $query_terms = array();

	/**
	 * Final tax query array (after merge)
	 */
	public $tax_query = '';

	/**
	 * @constructor
	 */
	public function __construct($post_data) {

		/**
		 * Create array of filter terms
		 * from search form $_POST data
		 */
		foreach($post_data as $key => $value){
			if($key != 'search' && $value != ''){

				/** 
				 * Add key/values to $query_terms array
				 * for persisting filter values in form
				 */
				$this->query_terms[$key] = htmlspecialchars($value);

				/** Create tax query $item from form */
				$this->item['taxonomy']  = htmlspecialchars($key);
				$this->item['terms']     = htmlspecialchars($value);
				$this->item['field']     = 'slug';

				/** Add $item to list */
				$this->list[] = $this->item;

			} elseif ($key == 'search' && $value != '') {

				/** Persist search term */
				$this->query_terms['search'] = htmlspecialchars($value);
			}
		}

		/** Create tax_query array after list merge with relation settings */
		if (count($this->list) > 0){
			$this->tax_query = array_merge(array('relation' => 'AND'), $this->list);
		}
	}
}

example.php

<?php
/*
 * Get search criterion from $_GET data
 */
$search_terms = new custom_search($_GET);
/*
 * Create arguments from search query
 */
$query_terms = $search_terms->query_terms;
$tax_query   = $search_terms->tax_query;
$paged       = ($query_terms->paged === 1) ? $query_terms->paged : ( get_query_var('paged') ) ? get_query_var('paged') : 1;
$args 		   = [
	's' 			       => $query_terms['search'],
	'post_type' 	   => 'custom_post_type',
	'posts_per_page' => '12',
	'order' 		     => 'ASC',
	'paged'			     => $paged,
	'tax_query'		   => $tax_query
];
/*
 * Get resources via WP_Query
 */
$custom_query = new WP_Query( $args );