I have this foo() method and I need to add unit test case for this method testFoo() in Magento. But Mage::getSingleton('checkout/cart') cannot be simulated.

Source method

public function foo()
{
    return Mage::getSingleton('checkout/cart')->getQuote()->getFunctionName();
}

Test function

public function testFoo(): void
{
    //$this->className :: Project_Catalog_Helper_Test
    $expected = 'string';
    $this->className->method('getSingleton')->willReturn($this->cart);
    $this->cart->method('getQuote')->willReturn($this->cart);
    $this->cart->method('getFunctionName')->willReturn('string');
    $this->assertEquals(
        $this->className->foo(),
        $expected
    );
}

When I run, php-unit receives this error log.

MacBook-Pro % ./vendor/bin/phpunit tests/src/app/code/local/project/Catalog/Helper/OrderTest.php
PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.11
Configuration: /project_path/phpunit.xml.dist

E                                                                   1 / 1 (100%)

Time: 150 ms, Memory: 8.00MB

There was 1 error:

1) Project_Catalog_Helper_Test::testFoo
Error: Call to a member function getCode() on bool

/project_path/app/code/core/Mage/Customer/Model/Session.php:103
/project_path/app/code/core/Mage/Core/Model/Config.php:1394
/project_path/app/Mage.php:517
/project_path/app/Mage.php:531
/project_path/app/code/community/Checkout/Model/Cart.php:20
/project_path/app/code/local/project/Catalog/Helper/Order.php:21
/project_path/tests/src/app/code/local/project/Catalog/Helper/OrderTest.php:100
/project_path/vendor/phpunit/phpunit/phpunit:53

Need some solutions, how to simulate Mage::getSingleton('checkout/cart').

Note: I cannot change the main method because it is legacy code. Just add unit test cases.