How do I code an if else PHP statement using sessions as conditions without getting error message?

I'm using the following php if statement as part of my code

if ($_SESSION['username'])

and everything is fine when the username session is set, but when it isn't, I get the following error message which I would like to not have show up.

Notice: Undefined index: username in C:\xampp\htdocs\mysites\ebay_tutorial\index.php on line 12

How can I improve my cod开发者_Python百科e so that the error message doesn't show up?

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This is happening because your PHP error reporting level is set to show Notice errors, which display if you try to access a key in an array which does not exist.

You can stop the error from being displayed by adding a call to isset:

if (isset($_SESSION['username']))