Display number of items in cart on the add to cart button

add_filter( 'woocommerce_product_add_to_cart_text', 'cssigniter_button_text_modifier' );
add_filter( 'woocommerce_product_single_add_to_cart_text', 'cssigniter_button_text_modifier' );
function cssigniter_button_text_modifier( $text ) {
	global $product;

	if ( ! $product ) {
		return $text;
	}

	$product_id = $product->get_ID();
	$quantities = WC()->cart->get_cart_item_quantities();
	$quantity   = 0;

	if ( empty( $quantities ) ) {
		return $text;
	}

	if ( 'simple' === $product->get_type() ) {

		if ( in_array( $product_id, array_keys( $quantities ), true ) ) {
			$quantity += $quantities[ $product_id ];
		}

		return $quantity > 0 ? $quantity . __( ' in cart', 'your-text-domain' ) : $text;
	} elseif ( 'variable' === $product->get_type() ) {
		$children = $product->get_children();

		foreach ( $children as $child => $id ) {
			if ( in_array( $id, array_keys( $quantities ), true ) ) {
				$quantity += $quantities[ $id ];
			}
		}

		return $quantity > 0 ? $quantity . __( ' in cart', 'your-text-domain' ) : $text;
	}

	return $text;
}

add_action('wp_footer', 'cssigniter_change_add_to_cart_text_js', 99);
function cssigniter_change_add_to_cart_text_js() {
	if ( ! is_woocommerce() ) {
		return;
	}
?>
	<script type="text/javascript">
		(function($) {
			$(document).on('click', '.ajax_add_to_cart, .remove_from_cart_button', function(e) {
				var clicked = $(this);
				var productId = $(this).data("product_id");
				var isRemoval = $(this).hasClass('remove_from_cart_button');
				var button = $(document).find('a[data-product_id=' + productId + '].add_to_cart_button');
				var singleButton = $('.single_add_to_cart_button');

				$.post("<?php echo admin_url( 'admin-ajax.php' ); ?>", {
						action: "cssigniter-add-to-cart-text-changer",
						id: productId,
						removal: true == isRemoval ? 1 : 0,
					})
					.done(function(data) {
						$(button).text(data);
						$(singleButton).text(data);
						if (isRemoval) {
							$(button).siblings('.added_to_cart').remove();
						}
					});
			});
		})(jQuery);
	</script>
<?php
}

add_action( 'wp_ajax_cssigniter-add-to-cart-text-changer', 'cssigniter_change_add_to_cart_text', 99 );
add_action( 'wp_ajax_nopriv_cssigniter-add-to-cart-text-changer', 'cssigniter_change_add_to_cart_text', 99 );
function cssigniter_change_add_to_cart_text() {
	$product_id = $_POST['id'];
	$is_removal = $_POST['removal'];
	$quantities = WC()->cart->get_cart_item_quantities();
	$product    = wc_get_product( $product_id );

	if ( 'simple' === $product->get_type() ) {
		$quantity     = 1;
		$default_text = __( 'Add to cart', 'your-text-domain' );

		if ( 0 === absint( $is_removal ) ) {
			if ( in_array( absint( $product_id ), array_keys( $quantities ), true ) ) {
				$quantity += $quantities[ $product_id ];
			}

			$quantity = $quantity . __( ' in cart', 'your-text-domain' );

			echo esc_html( $quantity );

		} else {
			echo esc_html( $default_text );
		}
	} elseif ( 'variable' === $product->get_type() && 1 === absint( $is_removal ) ) {
		$children     = $product->get_children();
		$default_text = __( 'Select options', 'your-text-domain' );
		$quantity     = 0;

		foreach ( $children as $child => $id ) {
			if ( in_array( $id, array_keys( $quantities ), true ) ) {
				$quantity += $quantities[ $id ];
			}
		}

		$quantity--;

		echo $quantity > 0 ? absint( $quantity ) . esc_html__( ' in cart', 'your-text-domain' ) : esc_html( $default_text );
	}

	wp_die();
}