function splitter($input) {
  return preg_split("[\\\\]", preg_replace('/((?:\&|\$)\w+;)/', "\\,$1,\\", $input));
}

print_r(splitter('Welcome to «PHP$raquo;!'));

Array
(
    [0] => Welcome to
    [1] => ,«,
    [2] => PHP
    [3] => ,$raquo;,
    [4] => !
)

// Исправленный вариант

function splitter($input) {
  return preg_split('[~]', preg_replace('/([^\&\$]+)((\&|\$)\w+;)/', "$1,~$2,~", $input));
}

var_dump(splitter('Welcome to «PHP$raquo;!'));

array(5) {
  [0]=>
  string(12) "Welcome to ,"
  [1]=>
  string(8) "«,"
  [2]=>
  string(4) "PHP,"
  [3]=>
  string(8) "$raquo;,"
  [4]=>
  string(1) "!"
}

function splitter($input) {
  return preg_split("[~]", preg_replace('/([^\&\$]+)(?:\&|\$)(\w+);/', "$1,~$2,~", $input));
}