print 'date(dny): ' . date(dny) . "\n";

$today = mktime(0, 0, 0, date("d")  , date("n"), date("y"));
print '$today: ' . $today . "\n";

$yesterday = $today - 86400;
print '$yesterday: ' . $yesterday . "\n";

$today = date(dny,$today);
$yesterday = date(dny,$yesterday);

print date(dny) . " $today $yesterday";




gives:
date(dny): 11906
$today: 1163055600
$yesterday: 1162969200
11906 091106 081106