I suspect you actually want to set the directories to 755 and either leave the files alone or set them to 644. The command can be used to do this find. For instance:

To change all directories to 755 (drwxr-xr-x):

find /opt/lampp/htdocs -type d -exec chmod 755 {} \;

To change all files to 644 (-rw-r--r--):

find /opt/lampp/htdocs -type f -exec chmod 644 {} \;