Summary: Problems fopen like parameter

Thank you Chris, you give me the clue Using your method discover wrong pair of brackets, that make that $url_rss become inside a function, so, the variable was look as local variable, not a global one; then, the value become null. - Blinded inside the box. Quoting Chris <dmagick@gmail.com>:

Maybe you're doing something to $url_rss between setting it and before the fopen call. echo $url_rss;