The class variable is declared outside of all other methods. To reference the variable use the following syntax.
Below is a brief example on how this would be used. The class variable is defined outside any functions, the variable is then set inside the constructor.
public function __construct()
$this->testVariable = 0;
This same syntax is the same for referencing functions within the same class.
Note: When calling the variable do not prefix with the $ sign as you would in other situations.