# 24 hours * 60 minutes/hour * 60 seconds/minute
$threshhold = 86400;
$currenttime = time();
if( ($currenttime - $mtime) > $threshhold){
    # file was modified within 24 hours
}

##</code><code>##

if( $mtime > ($currenttime - ($currenttime % 86400)){
   # file was modified this calendar day
}