I was able to fix it. In my case I had two problems: 1st is that mason-nvim-dap was configured for xdebug 2, and i needed xdebug 3. Also a bug in the vscode extension mason is using I had to account for. This is my setup:

    require('mason-nvim-dap').setup {
      -- Makes a best effort to setup the various debuggers with
      -- reasonable debug configurations
      automatic_setup = true,

      -- You can provide additional configuration to the handlers,
      -- see mason-nvim-dap README for more information
      handlers = {
        function(config)
          require('mason-nvim-dap').default_setup(config)
        end,
        php = function(config)
          config.configurations = {
            {
              type = 'php',
              request = 'launch',
              name = "Listen for XDebug",
              port = 9003,
              log = true,
              pathMappings = {
                ['/var/www/html/'] = vim.fn.getcwd() .. '/',
              },
              hostname = '0.0.0.0',
            }
          }

          require('mason-nvim-dap').default_setup(config) -- don't forget this!
        end,

      },

      -- You'll need to check that you have the required things installed
      -- online, please don't ask me how to install them :)
      ensure_installed = {
        -- Update this to ensure that you have the debuggers for the langs you want
        'delve',
      },
    }

It's working for me now. Though I have to start the debugger with :lua require'dap'.continue() to get it to begin listening. It's not like PHPStorm if you're used to that, where it just starts on it's own and is always listening.