Yesterday's time in PHP

$today=date("Y-m-d");
$yesterday = date('Y-m-d', mktime(0, 0, 0, date("m") , date("d") - 1, date("Y")));

1.  $today=date("Y-m-d"); 
2.  $yesterday = date('Y-m-d', mktime(0, 0, 0, date("m") , date("d") - 1, date("Y")));