Validation on Add To Cart button click action

cmnq_min_max_quantity.php

<?php

/*
* Validating the quantity on add to cart action with the quantity of the same product available in the cart. 
*/
function wc_qty_add_to_cart_validation( $passed, $product_id, $quantity, $variation_id = '', $variations = '' ) {

	$product_min = wc_get_product_min_limit( $product_id );
	$product_max = wc_get_product_max_limit( $product_id );

	if ( ! empty( $product_min ) ) {
		// min is empty
		if ( false !== $product_min ) {
			$new_min = $product_min;
		} else {
			// neither max is set, so get out
			return $passed;
		}
	}

	if ( ! empty( $product_max ) ) {
		// min is empty
		if ( false !== $product_max ) {
			$new_max = $product_max;
		} else {
			// neither max is set, so get out
			return $passed;
		}
	}

	$already_in_cart 	= wc_qty_get_cart_qty( $product_id );
	$product 			= wc_get_product( $product_id );
	$product_title 		= $product->get_title();
	
	if ( !is_null( $new_max ) && !empty( $already_in_cart ) ) {
		
		if ( ( $already_in_cart + $quantity ) > $new_max ) {
			// oops. too much.
			$passed = false;			

			wc_add_notice( apply_filters( 'isa_wc_max_qty_error_message_already_had', sprintf( __( 'You can add a maximum of %1$s %2$s\'s to %3$s. You already have %4$s.', 'woocommerce-max-quantity' ), 
						$new_max,
						$product_title,
						'<a href="' . esc_url( wc_get_cart_url() ) . '">' . __( 'your cart', 'woocommerce-max-quantity' ) . '</a>',
						$already_in_cart ),
					$new_max,
					$already_in_cart ),
			'error' );

		}
	}

	return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'wc_qty_add_to_cart_validation', 1, 5 );

/*
* Get the total quantity of the product available in the cart.
*/ 
function wc_qty_get_cart_qty( $product_id ) {
	global $woocommerce;
	$running_qty = 0; // iniializing quantity to 0

	// search the cart for the product in and calculate quantity.
	foreach($woocommerce->cart->get_cart() as $other_cart_item_keys => $values ) {
		if ( $product_id == $values['product_id'] ) {				
			$running_qty += (int) $values['quantity'];
		}
	}

	return $running_qty;
}

?>