Convert date to seconds in php8:
Step 1. Make sure timestamp is in English date format
Step 2. Make timestamp or date a string
Step 3. pass string to be parsed in strtotime()
Here is a few examples
Timestamp:
$str = '2022-07-05 08:36:33';
$timestamp = strtotime($str);
var_dump($timestamp); //1657010193
Date:
$str = '05 July 2022 08:36:33';
$date = strtotime($str);
var_dump($date); //1657010193