if you want to be fairly sure that your code is SQL injection safe against typical attacks, then you should use typical attacks to test with. Most hackers use an up-to-date bundle of tricks, typically already in a script, to try to cause harm...and don't bother hand hacking. As such, the trivial test cases presented do not represent a typical SQL injection hackers bundle, by any stretch of imagination.
also, to help prevent SQL injection...normally you also untaint the data by an inclusion regex. e.g.
bad_input() if($cityname !~ /^[a-zA-Z .,]+$/);
..and never untaint by disallowing banned characters instead. you never know if your banned character list is complete.
the hardest line to type correctly is: stty erase ^H
