This script allows you to indent HTML sections in PHP files. Create file ~/.vim/indent/php.vim ($HOME/vimfiles/indent/php.vim on Windows systems) containing the following:

" Better indent support for PHP by making it possible to indent HTML sections
" as well.
if exists("b:did_indent")
  finish
endif

" This script pulls in the default indent/php.vim with the :runtime command
" which could re-run this script recursively unless we catch that:
if exists('s:doing_indent_inits')
  finish
endif
let s:doing_indent_inits = 1
runtime! indent/html.vim
unlet b:did_indent
runtime! indent/php.vim
unlet s:doing_indent_inits

function! GetPhpHtmlIndent(lnum)
  if exists('*HtmlIndent')
    let html_ind = HtmlIndent()
  else
    let html_ind = HtmlIndentGet(a:lnum)
  endif
  let php_ind = GetPhpIndent()
  " priority one for php indent script
  if php_ind > -1
    return php_ind
  endif
  if html_ind > -1
    if getline(a:num) =~ "^<?" && (0< searchpair('<?', '', '?>', 'nWb')
          \ || 0 < searchpair('<?', '', '?>', 'nW'))
      return -1
    endif
    return html_ind
  endif
  return -1
endfunction

setlocal indentexpr=GetPhpHtmlIndent(v:lnum)
setlocal indentkeys+=<>>

CommentsEdit

I personally think it's more straightforward to just :set ft=html while you're editing HTML blocks.