How to work with variable functions and callbacks

How to pass one function to another

A variable function

      $function = (mt_rand(0,1) == 1) ? 'array_sum' : 'array_product'; 
      $values = array(4, 9, 16); 
      $result = $function($values); 
      

A function that uses a callback

      function validate($data, $functions) { 
        $valid = true; 
        foreach ($functions as $function) { 
          $valid = $valid && $function($data); 
        } return $valid; 
      } 
      function is_at_least_18($number) { 
        return $number >= 18; 
      } 
      function is_less_than_62($number) { 
        return $number < 62; 
      } 
      $age = 25; 
      $functions = array('is_numeric', 'is_at_least_18', 'is_less_than_62'); 
      $is_valid_age = validate($age, $functions); // true 
      

Language constructs that can’t be used in variable functions

      die eval echo empty exit
      include include_once isset list print 
      require require_once return unset
      

• A variable function is a function name stored in a variable as a string. When PHP encounters a variable function, it evaluates the variable and attempts to call the function.
• To call a variable function, code the variable name followed by a set of parentheses. Within the parentheses, code the argument list for the function.
• You can use a variable function when the function isn’t known until runtime.
• You can use a variable function in a function that uses a callback. A callback is a function that’s passed as an argument to another function.
• You can’t use the language constructs listed above with variable functions directly. However, you can use a wrapper function that calls one of these constructs in a variable function.

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