microtime for seconds

I'm checking how long a API takes to respond and I'm getting the values at start with microtime(true) and at the end with microtime(true) being so;

$start = microtime(true);
...
...
...
...
$end = microtime(true);

To convert to seconds the current code divides the subtraction of the values by 60:

($end - $start) / 60

But I did a lot of research and everything I find is about dividing the value by 1000000 , how do I get the difference of these values in segundos ?