Staying in the same php page!!! Help

[Sunking]

I really don't know how to do this.
On my page called index.php, users have to input their login and password.
When they click on submit, I have to check if the user name corresponds to the password.
I want to check that before going to the next page!
How can I check that by staying in the same page?

Basically, when they click on submit or "login", I don't want them to go to the other page if they have the wrong login and password.

Thank you

[planetsim]

if (!isset($_POST['submit'])) {
  //show main
} else {
  //validate
}

Hope that helps

[Sunking]

Hey thanks alot buddy
True, I am a beginner in php.