Do I need to know PHP to use WordPress?

No. You should be able to use WordPress through the user interface, without ever having to touch PHP.

The only time you would modify your WordPress website with PHP would be when integrating some of the plugins. There are a small number of plugins that still require manual edits to your files. In most cases, clear instructions are usually given within a text file with the plugin.

Other than that, you would not be changing any of the PHP files.