为什么asort适用于多维数组？

Question

Apologies if this is a really basic question. I've inadvertently discovered that asort() appears to work with multidimensional arrays:

Sample PHP

$animals = array(
  1 => array('name' => 'Zebra'),
  2 => array('name' => 'Fox'),
  3 => array('name' => 'Rabbit'),
  4 => array('name' => 'Dog'),
  5 => array('name' => 'Cat')
);

asort($animals);
var_dump($animals);

Output

array
  5 => 
    array
      'name' => string 'Cat' (length=3)
  4 => 
    array
      'name' => string 'Dog' (length=3)
  2 => 
    array
      'name' => string 'Fox' (length=3)
  3 => 
    array
      'name' => string 'Rabbit' (length=6)
  1 => 
    array
      'name' => string 'Zebra' (length=5)

I'd like to know why this works?

I thought asort() only sorted flat arrays, and to sort multidimensional arrays you needed to define a custom sort function. I can't find anything in the documentation that explains the above behaviour.

Answer 1

Arrays are compared in the lexicographical order by their values, so an array where the first element is "cat" is less than an array where the first element is "zebra". This is just a normal single dimension sort that happens to compare arrays to each other.

For example:

php > var_dump(array('a')< array('b'));
bool(true)
php > var_dump(array('a')> array('b'));
bool(false)