php-language-server doesn't run with global executable

## Information

**VIM version**

NVIM v0.3.1-dev
Build type: RelWithDebInfo

Operating System: Ubuntu 16.04

### :ALEInfo

Current Filetype: php
Available Linters: ['langserver', 'phan', 'php', 'phpcs', 'phpmd', 'phpstan']
Enabled Linters: ['langserver', 'phan', 'php', 'phpcs', 'phpmd', 'phpstan']
Suggested Fixers:
'php_cs_fixer' - Fix PHP files with php-cs-fixer.
'phpcbf' - Fix PHP files with phpcbf.
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
Linter Variables:

let g:ale_php_langserver_executable = '/home/candybar/.vim/vim-plugins/php-language-server/bin/php-language-server.php'
let g:ale_php_langserver_use_global = 1
let g:ale_php_phan_executable = 'phan'
let g:ale_php_phan_minimum_severity = 0
let g:ale_php_phan_use_client = 0
let g:ale_php_phpcs_executable = 'phpcs'
let g:ale_php_phpcs_options = ''
let g:ale_php_phpcs_standard = ''
let g:ale_php_phpcs_use_global = 1
let g:ale_php_phpmd_executable = 'phpmd'
let g:ale_php_phpmd_ruleset = 'cleancode,codesize,controversial,design,naming,unusedcode'
let g:ale_php_phpstan_configuration = ''
let g:ale_php_phpstan_executable = 'phpstan'
let g:ale_php_phpstan_level = '4'
Global Variables:

let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = 0
let g:ale_command_wrapper = ''
let g:ale_completion_delay = 100
let g:ale_completion_enabled = 1
let g:ale_completion_max_suggestions = 50
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 0
let g:ale_fixers = {}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'always'
let g:ale_lint_on_insert_leave = 0
let g:ale_linter_aliases = {}
let g:ale_linters = {}
let g:ale_linters_explicit = 0
let g:ale_list_window_size = 10
let g:ale_list_vertical = 0
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = -1
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 0
let g:ale_sign_error = '>>'
let g:ale_sign_info = '--'
let g:ale_sign_offset = 1000000
let g:ale_sign_style_error = '>>'
let g:ale_sign_style_warning = '--'
let g:ale_sign_warning = '--'
let g:ale_statusline_format = v:null
let g:ale_type_map = {}
let g:ale_use_global_executables = 1
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:

(executable check - failure) phan
(executable check - success) php
(finished - exit code 0) ['/usr/bin/zsh', '-c', 'php -l -d error_reporting=E_ALL -d display_errors=1 -d log_errors=0 -- < ''/tmp/nvimmCAR20/2/Plugin.php''']

<<>>
No syntax errors detected in -
<<>>

(executable check - failure) phpcs
(executable check - failure) phpmd
(executable check - failure) phpstan
(executable check - failure) phan
(finished - exit code 0) ['/usr/bin/zsh', '-c', 'php -l -d error_reporting=E_ALL -d display_errors=1 -d log_errors=0 -- < ''/tmp/nvimmCAR20/3/Plugin.php''']

<<>>
No syntax errors detected in -
<<>>

(executable check - failure) phpcs
(executable check - failure) phpmd
(executable check - failure) phpstan

## What went wrong

The language server does not run when editing php files. Linting via the php executable works fine.

The language server will run if it is installed with the project, in other words installed to the default location.

## Reproducing the bug

```vim
call plug#begin('~/.vim/vim-plugins')
Plug 'w0rp/ale'
let g:ale_php_langserver_use_global = 1
let g:ale_use_global_executables = 1
let g:ale_php_langserver_executable = expand('~/.vim/vim-plugins/php-language-server/bin/php-language-server.php')
let g:ale_completion_enabled = 1
Plug 'felixfbecker/php-language-server', {'do': 'composer install && composer run-script parse-stubs'}
call plug#end()
```
Installed w0rp/ale and felixbecker/php-language-server via vim-plug.
Set ale variables to enable global language server executable.
The language server executable is not called.

Apologies if I'm making some stupid mistake, but I've burned a couple hours on this now.