// Declaring class
class ExampleClass{
  const EXAMPLE_CONST_1 = 0;
  const EXAMPLE_CONST_2 = 1;

  function example_method($constant(?)){

     if($constant == ExampleClass::EXAMPLE_CONST_1)
       // do this
     else if($constant == ExampleClass::EXAMPLE_CONST_2)
       // do that

  }
}

// Using class
$inst = new ExampleClass();
$inst->example_method(ExampleClass::EXAMPLE_CONST_1);