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Yaf_Session::has() is a method in PHP's Yaf framework. It is used to determine if a specific key exists in the session data.

Here is an example that demonstrates the usage of Yaf_Session::has():

php
// Start a session
session_start();

// Set a value in the session
$_SESSION['username'] = 'John';

// Check if a key exists in the session
if (Yaf_Session::has('username')) {
echo 'Username is set in the session.';
} else {
echo 'Username is not set in the session.';
}


In this example, Yaf_Session::has('username') is used to check if the 'username' key exists in the session. If it does, the message "Username is set in the session." will be displayed; otherwise, the message "Username is not set in the session." will be displayed.

Note that Yaf_Session::has() is part of the Yaf framework and may not be available in other PHP frameworks or plain PHP projects.