find /var/www/html/dir/ -mtime -28 | grep '.php'

$ find /var/www/html/dir/ -type f -name "*.php" -newermt 2011-02-08 ! -newermt 2011-02-09

touch -t 201103070000 start.tmp
touch -t 201103080000 stop.tmp
find . -newer start.tmp ! -newer stop.tmp -print
rm start.tmp stop.tmp

find /var/www/html/dir/ -mtime 27 | grep '.php'