function yourfunction($a,$z){
   if($a<$z){ throw new <YOUR_EXCEPTION>; }
}

here is the test

class FunctionTest extends \PHPUnit_Framework_TestCase{

   public function testException(){

      $this->setExpectedException(<YOUR_EXCEPTION>::class);
      yourfunction(1,2);//add vars that cose the exception 

   }

}

function yourfunction($a,$z){
   if($a<$z){ throw new <YOUR_EXCEPTION>; }
}

here is the test

class FunctionTest extends \PHPUnit_Framework_TestCase{

   public function testException(){

      $this->setExpectedException(<YOUR_EXCEPTION>::class);
      yourfunction(1,2);//add vars that cause the exception 

   }

}