Automatically adding multiple products to the cart based on the multiple categories.

functions.php

<?php

/*
 * Automatically adding multiple products to the cart based on the multiple categories.
 */

function aaptc_add_product_to_cart( $item_key, $product_id ) {

	$product_category_id 	= array( 17, 18 ); // Category ids Hoodies & T-Shirts
	$product_cats_ids 		= wc_get_product_term_ids( $product_id, 'product_cat' ); // Getting assigned categories of product which is being added to cart
	
	$product_category_id_check = false;

	foreach ( $product_category_id as $key => $value ) {
		if ( in_array( $value, $product_cats_ids ) ) { // Checking if the specified category is being matched or not.
			$product_category_id_check = true;
		}
	}

    if ( ! is_admin() && $product_category_id_check ) {
        $free_product_ids = array( 122, 123 );  // Product Ids of the free products which will get added to cart e.g Gift A & Gift B

        foreach ( $free_product_ids as $pkey => $free_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() == $free_product_id )
	                    $found = true;
	                
	            }
	            // if product not found, add it
	            if ( ! $found )
	                WC()->cart->add_to_cart( $free_product_id );
	        } else {
	            // if no products in cart, add it
	            WC()->cart->add_to_cart( $free_product_id );
	        }
        }                
    }    
}
add_action( 'woocommerce_add_to_cart', 'aaptc_add_product_to_cart', 10, 2 );