I have an associative array with a whole pile of `true`/`false` values.

I am trying to remove all keys where the values are `false`.

So if the array is

```
array(
  'key1' => true,
  'key2' => false,
  'key3' => false,
  'key4' => true
);

```

I want to end up with

```
array(
  'key1' => true,
  'key4' => true
);

```

How would I do this?