Is it necessary to learn php in order to use drupal to manage my site?
No, it's not "necessary". Drupal core has a UI to perform its basic functions, as do most contributed modules.

Beyond that, it's totally up to you to say whether you need to learn PHP, based on the level of customisation you need in your site

If all the functionality you'll ever need can be provided through the UI of whatever modules you decide to use, then no, you won't need to learn PHP. If all of that functionality isn't provided "out-of-the-box", then yes, you'll need to learn PHP.

As with anything, your mileage will vary. The only possible way to answer this question properly is for you to get stuck in and try it out for yourself.