[line 44]
Keep track of state of the client's session with the server
Since there is no continuous connection between the client and the web server, there must be some way to carry information forward from one page to the next. PHP does this with a global array variable $_SESSION which is automatically restored from an area of the server's hard disk indicated by the contents of a cookie stored on the client's computer. This class is a static class with convenience methods for accessing the contents of $_SESSION.
A flash message is a message that will appear prominently on the next screen to be sent to the user. Flash messages are intended to be shown to the user once then erased. They are stored in the $_SESSION array for the user's session.
Tags:
return:
Content of the flash message with the specified key if $value is null; otherwise null.