Converting a PHP function to Objective-C and Java?

Converting a PHP function to Objective-C and Java?

I've written a function in PHP that I now need to write in Objective-C and
Java for an iOS and Android app respectively, but having issues with where
to even start.
The function is:
public static function decrypt($string, $key='my_key')
{
$result = '';
$string = str_replace(' ', '+', $string);
$string = base64_decode($string);
$string = str_replace($key, '', $string);
for($i=0; $i<strlen($string); $i++) {
$char = substr($string, $i, 1);
$keychar = substr($key, ($i % strlen($key))-1, 1);
$char = chr(ord($char)-ord($keychar));
$result.=$char;
}
return $result;
}
Result should return string:id
Anyone able to show me how to covert this into both languages? Any
guidance would be greatly appreciated.
Thanks.