Help with LSP-Mode for PHP with Intelephense

[Cheshire Yang]

Hi all,

I need help setting up emacs as a PHP IDE. I'm using Emacs 27 on NixOS.

First, it should be noted that MELPA works for me _very_ inconsistently, though 
the configuration and everything looks fine. I was not able to connect to MELPA 
today, so I cloned lsp-mode repository from github into .emacs.d/ and loaded it 
in the init file. So now, I can open a PHP file and M-x lsp-mode with success. 
I also installed Intelephense on the operating system, through the OS package 
manager (nix), and confirmed that I can type `intelephense` into a terminal and 
the command is available.

When I run `M-x lsp` to start the server however, I get a strange error. This 
is the output with toggle debug on error:

Debugger entered--Lisp error: (void-function -andfn)
  (-andfn #'lsp--supports-buffer\? #'lsp--server-binary-present\?)
  (lsp--filter-clients (-andfn #'lsp--supports-buffer\? 
#'lsp--server-binary-present\?))
  (let (clients (matching-clients (lsp--filter-clients (-andfn 
#'lsp--supports-buffer\? #'lsp--server-binary-present\?)))) (cond 
(matching-clients (if (setq lsp--buffer-workspaces (or (and (not ...) (lsp--tr$
  (progn (let (clients (matching-clients (lsp--filter-clients (-andfn 
#'lsp--supports-buffer\? #'lsp--server-binary-present\?)))) (cond 
(matching-clients (if (setq lsp--buffer-workspaces (or (and ... ...) (l$
  (if (buffer-file-name) (progn (let (clients (matching-clients 
(lsp--filter-clients (-andfn #'lsp--supports-buffer\? 
#'lsp--server-binary-present\?)))) (cond (matching-clients (if (setq 
lsp--buffer-workspac$
  lsp(nil)
  funcall-interactively(lsp nil)
  call-interactively(lsp record nil)
  command-execute(lsp record)
  execute-extended-command(nil "lsp" "lsp")
  funcall-interactively(execute-extended-command nil "lsp" "lsp")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

I have no clue what any of that means. Any and all help would be greatly 
appreciated. Unfortunately, the official docs on lsp-mode and intelephense are 
not useful.

Thank you,

C. Yang