What functions in PHP can be used to manipulate time and date values effectively?

When working with time and date values in PHP, the `date()` function is commonly used to format a timestamp into a desired date and time format. Additionally, the `strtotime()` function can be used to parse about any English textual datetime description into a Unix timestamp. These functions can be combined to effectively manipulate time and date values in PHP.

// Get the current date and time
$currentDateTime = date('Y-m-d H:i:s');

// Add 1 day to the current date
$nextDay = date('Y-m-d H:i:s', strtotime($currentDateTime . ' +1 day'));

echo $nextDay;