Array
(
    [first_name] => Abc
    [last_name] => Xyz
    [address] => xyz
    [email] => abcxyz@domain.com
    [contact_no] => 1234567890
    [exam_id] => 4
    [candidate_id] => abc.xyz
    [password] => seXUEz1n7cRRY
    [martial_status] => S
    [children] => 0
    [birth_date] => 1988-11-24
    [remark] => xyz
    [] => demo
)

$array = array('a' => 1, 'b' => 2, 'demo');
$index = array_search('demo', $array);

if ($index !== false) {
    $array['Father Name'] = $array[$index];
    unset($array[$index]);
}

var_dump($array);

$array["Father Name"] = $array[null];
unset($array[null]);

$array["Father Name"] = $array[''];
unset($array['']);

array('Father Name'=>'demo');