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/**
 * Use WC 2.0 variable price format, now include sale price strikeout
 *
 * @param  string $price
 * @param  object $product
 * @return string
 */
function wc_wc20_variation_price_format( $price, $product ) {
    // Main Price
    $prices = array( $product->get_variation_price( 'min', true ), $product->get_variation_price( 'max', true ) );
    $price = $prices[0] !== $prices[1] ? sprintf( __( 'From: %1$s', 'woocommerce' ), wc_price( $prices[0] ) ) : wc_price( $prices[0] );

    // Sale Price
    $prices = array( $product->get_variation_regular_price( 'min', true ), $product->get_variation_regular_price( 'max', true ) );
    sort( $prices );
    $saleprice = $prices[0] !== $prices[1] ? sprintf( __( 'From: %1$s', 'woocommerce' ), wc_price( $prices[0] ) ) : wc_price( $prices[0] );

    if ( $price !== $saleprice ) {
        $price = '<del>' . $saleprice . '</del> <ins>' . $price . '</ins>';
    }
    
    return $price;
}
add_filter( 'woocommerce_variable_sale_price_html', 'wc_wc20_variation_price_format', 10, 2 );
add_filter( 'woocommerce_variable_price_html', 'wc_wc20_variation_price_format', 10, 2 );