Test case:

$i = '-1';
$i++;
echo "i = $i\ntype = " . gettype($i) . "\n";

Output:
    i=-2            (****questionable behaviour****)
    type=string            (****questionable behaviour****)


Reference Case:

$i = '-1';
$i--;
echo "i = $i\ntype = " . gettype($i) . "\n";

Output:
    i=-2
    type=integer