How to get the difference between two datetime values

I want to get the difference between two datetime values and I don't know how,I tried to do this by making this simple way:
$difTime = $currTime - $logTime;

but it result "0",so I hope if there is a way to do it right.

Assuming datetime means they are formatted as `Y-m-d H:i:s`:

$difTime = strtotime( $currTime ) - strtotime( $logTime );

they formatted as `Y-m-d h:i:s`.
and thanks for help.