add_filter( 'woocommerce_get_availability', 'dispay_custom_icons_for_availability', 1, 2);
function dispay_custom_icons_for_availability( $availability, $product ) {
   global $product;

   // available
    if ( $product->is_in_stock() ) {
        $availability['availability'] = 'Stokta Var <i class="fa fa-lg fa-smile" style="color:green;"></i>';
        $availability['class'] = 'in_stock';
    }

    // middle stock
    if ( $product->is_in_stock() && $product->get_stock_quantity() <= 2 ) {
        $availability['availability'] = 'Stokta az kaldı <i class="fa fa-lg fa-meh" style="color:orange;"></i>';
        $availability['class'] = 'low_stock';
 }


   // out of stock
    if ( ! $product->is_in_stock() ) {
        $availability['availability'] = 'Stoklar Tükendi <img src="https://www.siteniz.com/stok.svg" class="stokk" width="25" height="25"></i>';
        $availability['class'] = 'out_of_stock';
    }

    return $availability;
}