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Passing a callback function as a parameter

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There are several PHP functions that accept user-defined callback functions as a parameter, such as: call_user_func(), usort() and array_map().

Depending on where the user-defined callback function was defined there are different ways to pass them:

Procedural style: 

function square($number)
{
    return $number * $number;
}

$initial_array = [1, 2, 3, 4, 5];
$final_array = array_map('square', $initial_array);
var_dump($final_array); // prints the new array with 1, 4, 9, 16, 25

Object Oriented style: 

class SquareHolder
{
    function square($number)
    {
        return $number * $number;
    }
}

$squaredHolder = new SquareHolder();
$initial_array = [1, 2, 3, 4, 5];
$final_array = array_map([$squaredHolder, 'square'], $initial_array);

var_dump($final_array); // prints the new array with 1, 4, 9, 16, 25

Object Oriented style using a static method: 

class StaticSquareHolder
{
    public static function square($number)
    {
        return $number * $number;
    }
}

$initial_array = [1, 2, 3, 4, 5];
$final_array = array_map(['StaticSquareHolder', 'square'], $initial_array);
// or:
$final_array = array_map('StaticSquareHolder::square', $initial_array); // for PHP >= 5.2.3

var_dump($final_array); // prints the new array with 1, 4, 9, 16, 25
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Functional programming:
* Functional Programming
* Closures
* Assignment to variables
* Objects as a function
* Using outside variables
* Using built-in functions as callbacks
* Anonymous function
* Pure functions
* Common functional methods in PHP
* Scope
* Passing a callback function as a parameter
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