WooCommerce Cart Validation. How to validate products being added to cart, before they are added.

functions.php

<?php

add_filter( 'woocommerce_add_to_cart_validation', 'yanco_validate_add_cart_item', 10, 5 );
function yanco_validate_add_cart_item( $passed, $product_id, $quantity, $variation_id = '', $variations = '' ) {

    $post_title = get_the_title( $product_id );

    // Items in cart, test if the product has already been added to the cart
    if ( sizeof( WC()->cart->get_cart() ) > 0 ) {

        foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
            $_product = $values['data'];

            // Test if it's a non-virtual product
            if( $_product->is_virtual() === false ) {
                if ( $_product->get_id() == $product_id ) {
                    $passed = false; // We found the item in the cart already
                }
            }
        }

    } else {
        // if no products in cart, passed = true
        $passed = true;
    }

    // do your validation, if not met switch $passed to false
    if ( $passed === false ) {
        wc_add_notice( __( '<strong>Du kan ikke tilføje ' . $post_title . ' til kurven da det allerede er lagt i kurven.</strong><br>Hvis du ønsker at tilføje tilvalg skal du fjerne ' . $post_title . ' fra kurven og tilføje det med tilvalg.<br>Hvis du ønsker at ændre antallet af kuverter skal du blot ændre antallet i kurven.', 'woocommerce' ), 'error' );
    }

    return $passed;

}