A callback function (often referred to as just "callback") is a function which is passed as an argument into another function.
Any existing function can be used as a callback function. To use a function as a callback function, pass a string containing the name of the function as the argument of another function:
Example:Pass a callback to PHP's array_map() function to calculate the length of every string in an array:
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Output :Array ( [0] => 3 [1] => 5 [2] => 5 [3] => 3 )
Callbacks in User Defined Functions :User-defined functions and methods can also take callback functions as arguments. To use callback functions inside a user-defined function or method, call it by adding parentheses to the variable and pass arguments as with normal functions:
Output :Hello world! Hello world?