I experience something strange with an undefined index..
$vatcode = 'U25';
echo $this->vatcode_ids['U25']."\n";
echo $this->vatcode_ids[$vatcode]."\n";
foreach($this->vatcode_account_ids as $id => $vatcode){
echo $vatcode."\n";
echo $this->vatcode_ids[$vatcode]; // undefined index
}
this returns:
681
681
U25
Notice: Undefined index: U25 in /var/www/.....php on line 64
I don't get it?!
From empty line printed before Notice massage i assume your $vatcode variable contains some ending new line character. If so it does not match any key in $this->vatcode_ids array.
You should use some trimming function as Dan Lee suggested in comments.