How to Pass Arguments by Value and Reference

An argument passed by value

      function add_3_by_val($value) {
        $value += 3; 
        echo("<p>Value: $value</p>\n");
      } 
      $number = 5;
      add_3_by_val($number);
      echo("<p>Number: $number</p>\n");

Value: 8

Number: 5

An argument passed by reference

      function add_3_by_ref(&$value) {

        $value += 3; 

        echo("<p>Value: $value</p>\n"); // Displays 8

      } 

      $number = 5; 

      add_3_by_ref($number); 

      echo("<p>Value: $value</p>\n"); // Displays 8
      Value: 8
Number: 8

How to modify a string that’s passed by reference

      function wrap_in_tag(&$text, $tag) {

        $before = '<' . $tag . '>'; 

        $after = ''; 

        $text = $before . $text . $after; 

      } 

      $message = 'Value out of range.'; 

      wrap_in_tag($message, 'p'); 

      echo("<p>$message</p>\n"); // Value out of range.
      Value out of range.

How to return multiple values

      function array_analyze($array, &$sum, &$prod, &$avg) { 

        $sum = array_sum($array); 

        $prod = array_product($array); 

        $avg = $sum / count($array);

      } 

      $list = array(1, 4, 9, 16); 

      array_analyze($list, $s, $p, $a); 

      echo("<p>Sum: $s Product: $p Average: $a</p>\n");
      Sum: 30 Product: 576 Average: 7.5

By default, all function arguments are passed by value. This means that a copy of the argument is sent to the function. As a result, when the function changes a parameter, it changes a copy of the argument, not the original argument.
To allow a function to modify the original argument instead of a copy, you can code an ampersand (&) in front of the parameter. In that case, the argument is passed by reference. This means that a reference to the original argument is sent to the function. Then, when the function changes the parameter, it’s actually changing the original argument.

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