PS wrong order, you are suppose to learn EVERYTHING in this list http://php.net/manual/en/langref.php you should never attempt to make your own first plugin without acquiring all the needed knowledge
If you call yourself a plugin developer and you don't know every single function documented on php.net, stop telling people to "learn everything of PHP".
You can learn at home, in your parents basement, in the garden, corner of the street, at school, at work, in your dreams, etc..... Just pick a spot that is comfortable and start!