PHP: set all values in an array to something

You can use array_fill_keys:

$array = array(
    1 => 231,
    2 => 432,
    3 => 324
);

$array = array_flip($array);

$array = array_fill_keys(array_keys($array), 1);

array_fill_keys() should be what you need:

$keys = array_keys($yourArray);
$filled = array_fill_keys($keys, 1);