What does “&” mean here in PHP?

Question

Consider this PHP code:

call_user_func(array(&$this, \'method_name\'), $args);

I know it means pass-by-reference when defining function

Answer 1

call_user_func(array(&$this, 'method_name'), $args);

This code generating Notice: Notice: Undefined variable: this

Here is a correct example:

The above example will output:

0
1