What should a junior PHP developer know to get a decent job?

Below are what I would suggest you know before applying:

• Understand plain PHP very very well. Know which functions to use when needed, super globals, file handling and among the other core PHP stuff.
• Understand Object Oriented Programming (OOP). This is because most of the company's code base will be written in OOP style.
• Know Structured Query Language (SQL) and understand it well. Though there are Non-SQL databases around I think almost every job uses SQL databases.
• Understand Model View Controller (MVC) and/or Model View View-Model (MVVM). These are the basic structure of all PHP Frameworks.
• After all, these, get to know at least one framework (I recommend Laravel). You don't need to master it, just know your "work-arounds" in it. Build some simple projects in them.
• Understand HTML and CSS. JavaScript is not compulsory for now but it will be a plus for you. And know some simple JQuery for DOM manipulation.

After you have gotten your first job:

• Be confident in yourself and don't be intimidated by Lead or Senior developers. But instead, learn from them and humble yourself. Know when to ask questions and when to work on your own. Be hard working and love your job. Most of the real experience in the developing world would be revealed to you in your job.
• Don't quit learning. Learn JavaScript (the most popular language in the world) and some of its frameworks. This will make you a full stack developer.
• Expand your territories by learning mobile development or anything outside your comfort zone.

if you want to explain more then write your thought in the comment section.