Automatically add product to cart on that product's page

opc_add_to_cart.php

<?php
/**
 * Automatically add product to cart on visit
 */
add_action( 'template_redirect', 'add_product_to_cart' );
function add_product_to_cart() {
  $product_ids = array( 128,132,140 ); //replace with your own product ids separated by commas
	foreach( $product_ids as $product_id) {
		if ( is_product($product_id) ) {
			$found = false;
			//check if product already in cart
			if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
				foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
					$_product = $values['data'];
					if ( $_product->get_id() == $product_id )
						$found = true;
				}
				// if product not found, add it
				if ( ! $found )
					WC()->cart->add_to_cart( $product_id );
			} else {
				// if no products in cart, add it
				WC()->cart->add_to_cart( $product_id );
			}
		}
	}
}