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

function keyprefix($keyprefix, Array $array) {

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

    return $array; 
}

Using array_flip will not preserve empty or null values. Additional code could be added in the unlikely event that the prefixed key already exists.