You've got the mtime, so you're already halfway there.
What you do with the mtime depends on what you mean by
"modified today".
If you mean within the last 24 hours, that's easy. Since
the mtime is second ssince the epoch,
# 24 hours * 60 minutes/hour * 60 seconds/minute
$threshhold = 86400;
$currenttime = time();
if( ($currenttime - $mtime) > $threshhold){
# file was modified within 24 hours
}
Or, if you mean was the file modified after the previous
12am, you have to calculate from the start of the caldendar
day with something like this
if( $mtime > ($currenttime - ($currenttime % 86400)){
# file was modified this calendar day
}