Why Has This Become 1 INT ? And Why Has That Become 0 INT ?

Hello,

$c = 'bool'; // boolean
settype($c, "integer"); // $c is now integer (1)
echo $c;

Why has $c become the int of 1 and not any other number ?
And why is the following set to an int of 0 and not any other number ?

$c = 'string'; // boolean
settype($c, "integer"); // $c is now integer (0)
echo $c;