Explain the ternary operator in the line $month = isset($month) ? $month : $today['mon']; in PHP.

The ternary operator in PHP is a shorthand way of writing an if-else statement. In the given line of code, the ternary operator is used to check if the variable $month is set. If it is set, the value of $month remains unchanged. If it is not set, the value of $today['mon'] is assigned to $month.

$month = isset($month) ? $month : $today['mon'];