PHP版：
$o = array('x'=>1, 'y'=>2, 'z'=>3);
$arr = array();
$i = 0;
foreach( $o as $arr[$i++]=>$v )/*empty*/;
var_dump($arr); //$arr = array('x', 'y', 'z');
JavaScrip版：
var o = {x:1, y:2, z:3};
var arr = [], i = 0;
for ( arr[i++] in o ) /*empty*/ ;
console.log( arr ); //arr = ['x', 'y', 'z'];