Fastest way to add prefix to array keys?

Question

What is the fastest way to add string prefixes to array keys?

Input

$array = array(
 \'1\' => \'val1\',
 \'2\' => \'val2\',
);

<

Answer 1

Could do this in one long line I presume:

$array = array_combine(
    array_map(function($k){ return 'prefix'.$k; }, array_keys($array)),
    $array
);

Or for versions of PHP prior to 5.3:

$array = array_combine(
    array_map(create_function('$k', 'return "prefix".$k;'), array_keys($array)),
    $array
);

There's probably dozens of ways to do this though:

foreach ($array as $k => $v)
{
    $array['prefix_'.$k] = $v;
    unset($array[$k]);
}