There are a lot of examples here for recursion that are meant to behave more like array_merge() but they don't get it quite right or are fairly customised. I think this version is most similar, takes more than 2 arguments and can be renamed in one place:
<?php
function array_merge_recursive_simple() {
    if (func_num_args() < 2) {
trigger_error(__FUNCTION__ .' needs two or more array arguments', E_USER_WARNING);
        return;
    }
$arrays = func_get_args();
$merged = array();
    while ($arrays) {
$array = array_shift($arrays);
        if (!is_array($array)) {
trigger_error(__FUNCTION__ .' encountered a non array argument', E_USER_WARNING);
            return;
        }
        if (!$array)
            continue;
        foreach ($array as $key => $value)
            if (is_string($key))
                if (is_array($value) && array_key_exists($key, $merged) && is_array($merged[$key]))
$merged[$key] = call_user_func(__FUNCTION__, $merged[$key], $value);
                else
$merged[$key] = $value;
            else
$merged[] = $value;
    }
    return $merged;
}
$a1 = array(
88 => 1,
'foo' => 2,
'bar' => array(4),
'x' => 5,
'z' => array(
6,
'm' => 'hi',
    ),
);
$a2 = array(
99 => 7,
'foo' => array(8),
'bar' => 9,
'y' => 10,
'z' => array(
'm' => 'bye',
11,
    ),
);
$a3 = array(
'z' => array(
'm' => 'ciao',
    ),
);
var_dump(array_merge($a1, $a2, $a3));
var_dump(array_merge_recursive_simple($a1, $a2, $a3));
var_dump(array_merge_recursive($a1, $a2, $a3));
?>
gives:
array(7) {              array(7) {              array(7) {
  int(1)                  int(1)                  int(1)
  ["foo"]=>               ["foo"]=>               ["foo"]=>
  array(1) {              array(1) {              array(2) {
    [0]=>                   [0]=>                   [0]=>
    int(8)                  int(8)                  int(2)
  }                       }                         [1]=>
  ["bar"]=>               ["bar"]=>                 int(8)
  int(9)                  int(9)                  }
  ["x"]=>                 ["x"]=>                 ["bar"]=>
  int(5)                  int(5)                  array(2) {
  ["z"]=>                 ["z"]=>                   [0]=>
  array(1) {              array(3) {                int(4)
    ["m"]=>                 [0]=>                   [1]=>
    string(4) "ciao"        int(6)                  int(9)
  }                         ["m"]=>               }
  [1]=>                     string(4) "ciao"      ["x"]=>
  int(7)                    [1]=>                 int(5)
  ["y"]=>                   int(11)               ["z"]=>
  int(10)                 }                       array(3) {
}                         [1]=>                     [0]=>
                          int(7)                    int(6)
                          ["y"]=>                   ["m"]=>
                          int(10)                   array(3) {
                        }                             [0]=>
                                                      string(2) "hi"
                                                      [1]=>
                                                      string(3) "bye"
                                                      [2]=>
                                                      string(4) "ciao"
                                                    }
                                                    [1]=>
                                                    int(11)
                                                  }
                                                  [1]=>
                                                  int(7)
                                                  ["y"]=>
                                                  int(10)
                                                }