PHP sort guidance required

As I’m not very strong on PHP, I thought I’d ask here for some help.

I’m trying to find a way sort an array in a wordpress theme (theme designers say they don’t provide help on customising themes).

The current default sort is set to post_date (as indicated in code below), I need to update the php to sort either by post_date or car_price or car_mileage or car_year, depending upon what has been selected on front page.

Problem is I haven’t the first notion of how I would go about doing this!

a demo is at demo

Here’s my partial code from the WP theme:

//*************************************************************************************************
$orderby_array = array(
	'post_date' => __( "Post date", 'cardealer' ),
	'car_price'     => __( "Price", 'cardealer' ),
	'car_mileage'   => __( "Mileage", 'cardealer' ),
	'car_year'      => __( "YOR", 'cardealer' ),
);

$orderby       = 'post_date'; <=== Change this value to anyone in the above array.

if ( isset( $_GET['orderby'] ) ) {
	$orderby = $_GET['orderby'];
}

$order = 'DESC';
if ( isset( $_GET['order'] ) ) {
	$order = $_GET['order'];
}

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	<div class="page-header sorting clearfix">

		<div class="sort-by"><?php _e( "Sort by:", 'cardealer' ) ?></div>

		<ul class="sort-by-list">

			<?php foreach ( $orderby_array as $key => $value ): ?>
				<li class="<?php echo ( $orderby == $key ) ? 'active' : '' ?>">
					<a href="javascript:void(0);"
					   class="js_order_cars_by order_cars_by search_order_<?php echo strtolower( $order ) ?>"
					   data-orderby="<?php echo $key ?>"
					   data-order="<?php echo( $order == 'DESC' ? 'ASC' : 'DESC' ) ?>" <?php echo strtoupper( $value ) == 'YOR' ? 'title="Year of registration"' : ''; ?>>
						<?php echo $value ?>
					</a>
				</li>
			<?php endforeach; ?>

		</ul>
		<!--/ .sort-by-list-->