What is the simplest way to convert a timestamp stored in a variable to a readable date format in PHP?

To convert a timestamp stored in a variable to a readable date format in PHP, you can use the `date()` function along with the `strtotime()` function. The `strtotime()` function converts the timestamp to a Unix timestamp, which can then be formatted using the `date()` function with the desired date format.

$timestamp = 1609459200; // Example timestamp
$date = date('Y-m-d H:i:s', strtotime($timestamp));
echo $date; // Output: 2021-01-01 00:00:00