Invalid UTF-8 Sitemap contains CO or C1 control codes

I came across "Invalid UTF-8 Sitemap contains CO or C1 control codes" in WMT tools the other day. The sitemaps had recenlty been changed from a normal to (the compressed version).

I looked through the uncompressed version of the sitemap and didn't see anything out of the ordinary characters-wise. When I ran it through some UTF-8 checks it came back as valid, so I was at a loss as to what was causing the error.

What do you do when you need some answers? Google it! The only post I found with that error message had 0 replies and it wasn't posted by someone having a PHP problem. After broadening my search a little bit I came across this useful stackoverflow post that alluded it was because fopen/fclose were appending characters that were not being encoded. Switching to gzopen, gzwrite, and gzclose worked perfect for me. gzwrite is binary-safe, so best guess would be that fwrite was including a PHP_EOL in the file that was resulting in the error.

$xml = '<!--?xml version="1.0" encoding="UTF-8"?-->
$gz = gzopen('sitemap.xml.gz','w9');
gzwrite($gz, $xml);