There are a number of special keywords you can place before a class, a class function definition, or a class variable to alter the way PHP works with it - here's the full list, along with what each of them does:
Public: This variable or function can be used from anywhere in the script
Private: This variable or function can only be used by the object it is part of; it cannot be accessed elsewhere
Protected: This variable or function can only be used by the object it is part of, or descendents of that class
Final: This variable or function cannot be overridden in inherited classes
Abstract: This function or class cannot be used directly - you must inherit from them first
That is just a vague description of what each of them do - to make sure you fully understand each of them, here are examples:
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