php - How to mock inner function in phpunit?

one text

This is my Foo class

<?php


class FooClass
{
    public function doFoo($total)
    {
        $gst = (($this->getTaxRate() / 100) * $total);

        return $gst;
    }

    public function getTaxRate()
    {
        return 5;
    }
}

And this is my test.

public function fooTest()
{
    $mockedObj = $this->createMock(FooClass::class);

    $mockedObj->expects($this->exactly(1))
        ->method('getTaxRate')
        ->will($this->returnValue(10));

    $this->assertEquals(10, $mockedObj->doFoo(100));
}

Output:

Failed asserting that null matches expected 10.

I can see getTaxRate() function is not being mocked .It is also returning null instead of 5(default value). I am expecting getTaxRate() inside doFoo() to return the mock value i.e 10 else if I do not mock a getTaxRate, it should return 5.

Anything that I am missing here?