maintenance?

To redirect all page requests to a temporary page, just add this to your .htaccess file.

Just make sure your own IP is in the 3rd line. Make a copy of that line to add additional addresses as well.

Also, this code assumes that your temporary page is located at /not-up.htm. If it's not there, you will have to change those last two lines accordingly.

Method I use: For php home page, put in an html page and use a .htaccess file to give html priority, if necessary. For an html home page, add a php page.

Or, alternatively, rather than remove users access to the site when you work on it, you can "mirror" (copy) your entire site for working on.

So you would work on the mirrored site and when you have completed something and fully tested it to confirm there are no errors, you can then simply copy it to the live site. Users might get a little disruption, but only seconds.

All this depends on what you work on of course. If you are working on something where you need to stop users inputting or using a function as you need to take the current data from live and change it to a new procedure, then you would not want users inputting more data in "the old way", so then you might still want to "close" the site for that.
But general improvements/maintenance etc can be done on a mirror site and avoiding the downtime.

And even if you need to shut down the site just to upload new code, working on the mirrored site makes downtime a few seconds, rather than the time you are making/editing code as well.