Question

I was reading through the code and came across this syntax:

$customerIDs = array_map(function(Customer $customer) { return $customer->id; }, $customers);

where $customers is the array.

My confusion is in trying to understand function(Customer $customer). I see that Customer is a class, but what is $customer then?

Answer 1

This part:

function(Customer $customer) { return $customer->id; }

is an anonymous function. It’s a "new" feature in PHP 5.3. It’s pretty much equivalent to:

function someFunction(Customer $customer) {
    return $customer->id;
}

$customerIDs = array_map('someFunction', $customers);

As for the Customer $customer part, that’s just a type-constrained argument. It throws an error if the argument passed is not of type Customer.

You can read more about anonymous functions at the php.net documentation.