gistfile1.txt

fredemmott-mbp1:~ fredemmott$ cat test.php
<?php

$keys = array(3.141);
var_dump(array_fill_keys($keys, 'value'));

$foo = array();
$foo[3.141] = 'value';
var_dump($foo);
fredemmott-mbp1:~ fredemmott$ php test.php
array(1) {
  ["3.141"]=>
  string(5) "value"
}
array(1) {
  [3]=>
  string(5) "value"
}
fredemmott-mbp1:~ fredemmott$ php --version
PHP 5.4.17 (cli) (built: Sep 12 2013 23:14:23)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies