In PHP, callback is a function object/reference with type callable. A callback/callable variable can act as a function, object method and a static class method. There are various ways to implement a callback. Some of them are discussed below:
Standard callback: In PHP, functions can be called using call_user_func() function where arguments is the string name of the function to be called.
Static class method callback: Static class methods can be called by using call_user_func() where argument is an array containing the string name of class and the method inside it to be called.
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Object method callback: Object methods can be called by using call_user_func() where argument is an array containing the object variable and the string name of method to be called. Object method can also be called if they are made invokable using __invoke() function definition. In this case, argument to the call_user_func() function is the object variable itself.
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Closure callback: Closure functions can be made callable by making standard calls or mapping closure function to array of valid arguments given to the closure function using array_map() function where arguments are the closure function and an array of its valid arguments.
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