$ /usr/local/php5/bin/php
<?php

$n = 1;
echo("INDEX: " . $n . "\n");
echo("INDEX: " . $n-1 . "\n");
?>
INDEX: 1
-1



$ /usr/local/php4/bin/php
<?php

$n = 1;
echo("INDEX: " . $n . "\n");
echo("INDEX: " . $n-1 . "\n");
?>

INDEX: 1
-1