public function testConvertCurrency() { $expectedResult = 5; $mock = $this->getMockBuilder(CurrencyConverter::class) ->setMethods(['convert']) ->getMock(); $mock->expects($this->once()) ->method('convert') ->with('USD', 'AUD') ->willReturn($expectedResult); $conversionResult = $mock->convertCurrency('USD', 'AUD', 1); $this->assertEquals($expectedResult, $conversionResult); }In this code example, the _set() method is used to set up an expectation for the CurrencyConverter object's convert() method. It is expected to receive two arguments - 'USD' and 'AUD'. The method will return the expected result - 5. The code above uses the PHP package PHPUnit_Framework_MockObject, which is a part of the PHPUnit testing framework. This library provides users with easy-to-use tools for creating mock objects and defining expectations for them during testing.