When you are unit testing you should be trying to test your method under test (unit of work under test) in isolation from the rest of your system. As soon as you are forced to cross a system boundary, your unit test becomes an integration test. Integration tests tend to be more fragile, harder write and harder to maintain. By isolating the mailing component from the method that creates the mail you no longer need to rely on whether the mailing system is up and running when you unit test runs. You are free to test the logic in the method under test without having to depend on a concrete implementation.
There are two categories of mock tests, interaction tests and state based tests. I'd suggest looking up the difference between the two on google.
I am unfortunately not familiar with the syntax of php to I can only show you an example of what you might be testing in your case using C# syntax, but I am sure you will be able to follow the code.
public interface Mailer
{
void Send(string to, string from, string subject, string body)
}
The above interface would be one that matches the signature of your mailing component that actually sends the mail (it knows about smtp etc).
public class MailSender
{
private IMailer _mailer;
public MailSender(IMailer mailer)
{
_mailer = mailer;
}
public void GenerateMail(string to, string cc, string from, string subject, string body)
{
if (IsValidEmailAddress(to) == false) throw new InvalidEmailAddressException("To");
if (IsEmptySubject(from) == false) throw new EmptySubjectException();
_mailer.Send(to, from, subject, body);
}
private bool IsValidEmailAddress(string emailAddress)
{
// some code/regex to check that the email address is valid
}
private boold IsEmptySubject(string subject)
{
// code to check if the subject is empty
}
}
Now we have logic in the GenerateMail method. You can write unit tests to check if you get exceptions for invalid email addresses and an empty subject and you can also write a unit test to ensure that if you have valid email addresses and a subject that is not empty the send method is called on the mocked mail sender with the correct information in each of the parameters. This type of unit test would an interaction test as the mock is just verifying that the method was called).
Hope this explanation helps.
