How can you convert a date into a timestamp in PHP for calculations?

In PHP, you can convert a date into a timestamp using the strtotime() function. This function will parse the date string and return the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT). This timestamp can then be used for calculations such as date differences or comparisons.

$date = "2022-01-01";
$timestamp = strtotime($date);

echo $timestamp;