The next "language" or "framework" or "library" that you should learn depends on the kind of Web Developer you want to become...::
Front End Developer
• Ruby on Rails
I advise that you pick the ones according to your interests...
Front-end : Angular.js , React.js , Bootstrap.js
Back-end : Node.js , Symfony.php , Django.py
I also recommand Java and Linux for server config
Edit : find nice tutorials on W3Schools, and use framework documentation.
also, if you don't know PHP, go !
Even I suggest you w3 schools for amazing courses on HTML , CSS , php and SQL .
Moreover tutorials point is also an amazing website and application for learning . They even have many PDF . Almost everything mentioned by countable.n are present on tutorials point . Further I'll suggest you to like learn THREE.js after learning that all .
learn PHP for back-end and the SQL for database operations
then one will be perfect to develop a web application. One can earn by developing fee lance projects.
To get paid a bit more learn JQUERY which can bring animations to project
To reduce CSS work learn BOOTSTRAP
To reduce work and time for project development use frameworks - CODEIGNITER is framework based on PHP
All the best.
Okay that does it... all you PHP lovers, read this...
PHP sucks... period
Php frameworks (laravel,codeignitor,cakephp) and python framework (django) will be great addition in a web developer's skill set. Rest is as everyone says : php,sql,js libraries,bootstrap (python in case you wanna take your skills on even higher levels)