What are some common functions in PHP for converting dates and times to Unix timestamps?

When working with dates and times in PHP, it is common to convert them to Unix timestamps for easier manipulation and comparison. PHP provides several functions for converting dates and times to Unix timestamps, such as strtotime() and mktime(). These functions can parse date and time strings and return the corresponding Unix timestamp.

// Convert a date string to a Unix timestamp using strtotime()
$dateString = "2022-01-01";
$timestamp = strtotime($dateString);
echo $timestamp;

// Convert a specific date and time to a Unix timestamp using mktime()
$timestamp = mktime(12, 0, 0, 1, 1, 2022);
echo $timestamp;