What is the system variable equivalent to SQL datetime in PHP, if any?

In PHP, the system variable equivalent to SQL datetime is the `date()` function. This function allows you to format a timestamp into a date string that is compatible with SQL datetime format. You can use the `date()` function with the 'Y-m-d H:i:s' format specifier to get the current date and time in SQL datetime format.

// Get the current date and time in SQL datetime format
$sqlDatetime = date('Y-m-d H:i:s');
echo $sqlDatetime;