WooCommerce: Check if items are already in cart

global $woocommerce and $woocommerce->cart is outdated and simply replaced by WC()->cart

Here is a custom function with an argument that accepts a unique integer product ID or an array of product IDs, and that will return the number of matched Ids that are in cart.

The code handle any product type, including variable product and product variations:

function matched_cart_items( $search_products ) {
    $count = 0; // Initializing

    if ( ! WC()->cart->is_empty() ) {
        // Loop though cart items
        foreach(WC()->cart->get_cart() as $cart_item ) {
            // Handling also variable products and their products variations
            $cart_item_ids = array($cart_item['product_id'], $cart_item['variation_id']);

            // Handle a simple product Id (int or string) or an array of product Ids 
            if( ( is_array($search_products) && array_intersect($search_products, cart_item_ids) ) 
            || ( !is_array($search_products) && in_array($search_products, $cart_item_ids)
                $count++; // incrementing items count
        }
    }
    return $count; // returning matched items count 
}

This code goes in function.php file of your active child theme (active theme or in any plugin file).

Code is tested and works.

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USAGE:

1) For a unique product ID (integer):

$product_id = 102;

// Usage as a condition in an if statement
if( 0 < matched_cart_items($product_id) ){
    echo '<p>There is "'. matched_cart_items($product_id) .'"matched items in cart</p><br>';
} else {
    echo '<p>NO matched items in cart</p><br>';
}

2) For an array of product IDs:

$product_ids = array(102,107,118);

// Usage as a condition in an if statement
if( 0 < matched_cart_items($product_ids) ){
    echo '<p>There is "'. matched_cart_items($product_ids) .'"matched items in cart</p><br>';
} else {
    echo '<p>NO matched items in cart</p><br>';
}

3) For an array of product IDs for 3 or more matched cart items for example:

$product_ids = array(102, 107, 118, 124, 137);

// Usage as a condition in an if statement (for 3 matched items or more)
if( 3 <= matched_cart_items($product_ids) ){
    echo '<p>There is "'. matched_cart_items($product_ids) .'"matched items in cart</p><br>';
} else {
    echo '<p>NO matched items in cart</p><br>';
}