On the QaFoo.com blog they've made a recommendation in their latest post - they suggest that you never use null.

When doing code reviews together with our customers we see a pattern regularly which I consider problematic in multiple regards – the usage of null as a valid property or return value. We can do better than this.

Let's go into common use cases first and then discuss how we can improve the code to make it more resilient against errors and make it simpler to use. Most issues highlighted are especially problematic when others are using your source code. As long as you are the only user (which hopefully is not the case) those patterns might be fine.

They talk about some of the most common uses they see for using null in PHP applications including setters for class properties (injection). They point out that in PHP 7 a missing value on a property would result in a Fatal error and make the functionality harder to test overall. They suggest that all required dependencies be injected on object construction instead, making it easier to know the current state of the object on testing. They also talk some about using null as a return value, how it could make debugging difficult and a solution that could make more sense - throwing an exception instead.