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]);
}