<?php
$dayString = "2011-02-12 14:44:00";
$dayStringSub = substr($dayString, 0, 10);

$isToday = ( strtotime('now') >= strtotime($dayStringSub . " 00:00") 
          && strtotime('now') <  strtotime($dayStringSub . " 23:59") );
echo "checked: $dayString, \tresult: $isToday";