Wishing for it wont make it come true! Unfortunately it is this type of reply that gives PHP and freelance people a bad name. Yes PHP is opensource the same as MySQL is but how many people really go in and change the source code of for PHP or MySQL itself? All of the listed serverside "languages" can have both opensource and close source scripts/applications written in them, which is what wplive really is talking about, but that is totally independant of if the language itself is opensource or not. To the original question, there is no best one, if there was a best one then there would only be one. The reality is that each have their own strengths and weaknesses. If you search this forum you will find 100+ threads discussing what these are
PHP is a BIG thing to go with! However, Personally, I like changes so I am changing to track of Ruby on Rails! PHP is still the BEST one IMO to start with.
Opensource isn't just an advantage if you personally want to dive in and change code. There's choice of development tools, community, documentation etc
PHP will easily be mentioned most. But I do believe Python and Ruby on Rails are superior. If you've enough experience with all of them, you'll know what I mean. PHP is just too old for modern web Dev.