You should focus in PHP 7.0, PHP 5.5 stopped receiving support the last week, and PHP 5.6 will stop getting active support at the end of this year, so it's not worth the trouble to learn them.
Although I hadn't seen or heard about a good resource "from zero to PHP 7.0", the most I can recommend you is the Modern PHP book, but it assumes the reader has some familiarity with pre-5.4 PHP (ie. non-modern PHP).
Although it is still a suggested read when you become acquainted with the language, since it introduces you to the Composer ecosystem, the PHP-FIG Standards, the PHP Right Way, and some other gems of information.
EDIT: I did a quick search, these books seem like the most sensible choice right now:
I just skimmed through the table of contents of them, the first one looks more focused in starting from zero, the second one looks more focused in creating CRUD applications, which is the most common kind of application in the PHP world, since I hadn't read them, I can't give a recommendation, but I would suggest to read both of them, the Sklar's one first.