To convert a datetime to a timestamp in PHP, you can use the strtotime() function. The formula to convert datetime to timestamp is as follows:

$timestamp = strtotime(#p#);
return $timestamp;

Replace `#p#` with your desired datetime value in the format of day/month/year (e.g., 10/12/2022). The formula will convert the given datetime to its corresponding timestamp and return the result.

Please note that this formula assumes you have already set up your PHP environment and have valid input for the `#p#` placeholder.