Post Filtering

category.php

global $wp_query;
$modifications = array();
if( !empty( $_GET['catname'] ) ) {
	$modifications['category_name'] = $_GET['catname'];
}

$args = array_merge( 
	$wp_query->query_vars, 
	$modifications 
);

query_posts( $args );

child-functions.php

add_action( 'wp_enqueue_scripts', 'enqueue_parent_styles' );
function enqueue_parent_styles() {
    wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );
}

child-style.css

/*
 Theme Name:   Twenty Fifteen With Post Filter
 Theme URI:    http://yourwebsite.com/twentyfifteen-postfilter/
 Description:  My post filter project, based on Twenty Fifteen
 Author:       Daniel Pataki
 Author URI:   http://danielpataki.com
 Template:     twentyfifteen
 Version:      1.0.0
 Tags:         black, green, white, light, dark, two-columns, three-columns, left-sidebar, right-sidebar, fixed-layout, responsive-layout, custom-background, custom-header, custom-menu, editor-style, featured-images, flexible-header, full-width-template, microformats, post-formats, rtl-language-support, sticky-post, theme-options, translation-ready, accessibility-ready, responsive-layout, infinite-scroll, post-slider, design, food, journal, magazine, news, photography, portfolio, clean, contemporary, dark, elegant, modern, professional, sophisticated
 Text Domain:  twenty-fourteen-child
*/

filter-styles.css

.post-filters {
	margin: 0 8.3333% 8.3333% 8.3333%;
	box-shadow: 0 0 1px rgba(0, 0, 0, 0.15);
	padding:22px;
	background: #fff;
}

filters.html

<div class='post-filters'>
	<select name="orderby">
		<option value='post_date'>Order By Date</option>
		<option value='post_title'>Order By Title</option>
		<option value='rand'>Random Order</option>
	</select>
	<select name="order">
		<option value='DESC'>Descending</option>
		<option value='ASC'>Ascending</option>
	</select>
	<select name="thumbnail">
		<option value='all'>All Posts</option>
		<option value='only_thumbnailed'>Posts With Thumbnails</option>
	</select>
</div>

form-loop.php

<form class='post-filters'>
	<select name="orderby">
		<?php
			$orderby_options = array(
				'post_date' => 'Order By Date',
				'post_title' => 'Order By Title',
				'rand' => 'Random Order',
			);

			foreach( $orderby_options as $value => $label ) {
				echo "<option ".selected( $_GET['orderby'], $value )." value='$value'>$label</option>";
			}
		?>
	</select>
	<select name="order">
		<?php
			$order_options = array(
				'DESC' => 'Descending',
				'ASC' => 'Ascending',
			);

			foreach( $order_options as $value => $label ) {
				echo "<option ".selected( $_GET['order'], $value )." value='$value'>$label</option>";
			}
		?>
	</select>
	<select name="thumbnail">
		<?php
			$order_options = array(
				'all' => 'All Posts',
				'only_thumbnailed' => 'Posts With Thumbnails',
			);

			foreach( $order_options as $value => $label ) {
				echo "<option ".selected( $_GET['thumbnail'], $value )." value='$value'>$label</option>";
			}
		?>
	</select>
	<input type='submit' value='Filter!'>
</form>

incorrect-selected.php

<option value='post_date' <?php if( ( !empty( $_GET['orderby'] ) && $_GET['orderby'] == 'post_date' ) || empty( $_GET['orderby'] ) ) echo "selected='selected'" ?>>Order By Date</option>
<option value='post_title'  <?php if( ( !empty( $_GET['orderby'] ) && $_GET['orderby'] == 'post_title' ) ) echo "selected='selected'" ?>>Order By Title</option>
<option value='rand' <?php if( ( !empty( $_GET['orderby'] ) && $_GET['orderby'] == 'rand' ) ) echo "selected='selected'" ?>>Random Order</option>

orderby-loop.php

<select name="orderby">
	<?php
		$orderby_options = array(
			'post_date' => 'Order By Date',
			'post_title' => 'Order By Title',
			'rand' => 'Random Order',
		);

		foreach( $orderby_options as $value => $label ) {
			echo "<option ".selected( $_GET['orderby'], $value )." value='$value'>$label</option>";
		}
	?>
</select>

query.php

global $wp_query;
$modifications = array();
if( !empty( $_GET['catname'] ) && $_GET['thumbail'] == 'only_thumbnailed' ) {
	$modifications['meta_query'][] = array(
		'key' => '_thumbnail_id',
		'value' => '',
		'compare' => '!='
	);
}

$args = array_merge( 
	$wp_query->query_vars, 
	$modifications 
);

query_posts( $args );