create stand-alone PHP page for site

I want to create an “About” page that allows me to put PHP code in the content. For example, such as page might be located as https://example.com/about.php on my site.

The “admin -> manage -> pages” does not allow for this.

So I created “about.php” in “wp-content/themes/example/about.php” modeling single.php.

But I’m getting “Page not found” when I go to https://example.com/about.php

And I’m getting “Can not call undefined function get_header” when I go to https://example.com/wp-content/themes/example/single.php

Any idea how to create a stand-alone php page for the site that uses the templating system?