What is the static keyword in PHP?

The static keyword in PHP has multiple uses. Static class members (properties and methods) belong to the class rather than an instance — access with ClassName::$property or ClassName::method(). Static variables inside functions retain their value between function calls: function counter() { static $count = 0; return ++$count; }. Late static binding (using static:: instead of self::) refers to the class that was actually called at runtime, not where the method was defined — crucial for inheritance scenarios where you want to reference the child class from a parent method. self:: always refers to the class where the code is written; static:: refers to the runtime class.