Change value of PHP array with Condition

$file = array();

$file[0] = 0;
$file[1] = "VietNam";
$file[2] = "";

$file = array_map(function($v) { if ($v == 0 || $v == "") {return "NULL";} return $v;}, $file);

/* result
    $file[0] = "NULL";
    $file[1] = "VietNam";
    $file[2] = "NULL";
*/