$multi = array(
  array(
    array(
      'wo',
      'shi'
    ),
    'php'
  ),
  'cheng',
  array(
    array(
      'xu',
      'yuan',
    )
  ),
  '!'
);
$multi = arrToOne($multi);
print_r($multi);
function arrToOne($multi) {
  $arr = array();
  foreach ($multi as $key => $val) {
    if( is_array($val) ) {
      $arr = array_merge($arr, arrToOne($val));
    } else {
      $arr[] = $val;
    }
  }
  return $arr;
}

Array
(
 [0] => wo
 [1] => shi
 [2] => php
 [3] => cheng
 [4] => xu
 [5] => yuan
 [6] => !
)