Access array element within same array

Is this possible in PHP?

$a = array('first' => 'some value', 'second' => $a['first']);

Simply accessing an开发者_运维问答 array element within another element from the same array.

Not with this syntax, because at the time $a['first'] is evaluated, $a is undefined. This would work though:

$a = array();
$a['first'] = 'some value';
$a['second'] = $a['first'];