Difference between $this->a and $this->$b in PHP
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What is the difference between:

`$this->$a` and `$this->b`

In my class I have this:

    class someClass{
        public $a;
        function aFunction(){
          $this->a = 5;
          $this->$b = 7;
        }
     }

what does having the extra '$' do in the `$this->$b`

