Using Pass-by-Value Arguments for References

PHP Tutorials - Herong's Tutorial Examples

∟Function Declaration, Arguments, and Return Values

∟Using Pass-by-Value Arguments for References

This section provides a tutorial example on how to references on pass-by-value arguments. Using a reference expression as a pass-by-value argument allows you to modify the original variable in the calling code.

In the previous section, we tested on passing regular variables to functions using the pass-by-value mode. But what will happen if we pass references to functions using the pass-by-value mode? To find out the answer, I wrote this tutorial example script:

<?php 
#  PassByValueWithReference.php
#- Copyright (c) 2003-2019, HerongYang.com, All Rights Reserved.
# 
function swap($left, $right) {
   $temp = $left;
   $left = $right; 
   $right = $temp;
   print("    Swapped in function: ".getString($left, $right)."\n");
}

   print("\n 1. Passing two variables:\n");
   $x = "Dog"; $y = "Cat";
   print("    Before call: ". getString($x, $y) ."\n");
   swap($x, $y);
   print("    After call: ". getString($x, $y) ."\n");

   print("\n 2. Passing two references:\n");
   $y = "Bob"; $y = "Tom";
   print("    Before call: ". getString($x, $y) ."\n");
   swap(&$x, &$y);
   print("    After call: ". getString($x, $y) ."\n");

   print("\n 3. Passing two references in variables:\n");
   $a = "One"; $b = "Two";
   $x = &$a; $y = &$b;
   print("    Before call: ". getString($a, $b) ."\n");
   print("    Before call: ". getString($x, $y) ."\n");
   swap($x, $y);
   print("    After call: ". getString($a, $b) ."\n");
   print("    After call: ". getString($x, $y) ."\n");

function getString($left, $right) {
   if (is_scalar($left)) {
      return "$left | $right";
   } else {
      return NULL;
   }
}
?>

If you run this tutorial example with older version of PHP, you will get:

C:\herong> \php\php PassByValueWithReference.php

 1. Passing two variables:
    Before call: Dog | Cat
    Swapped in function: Cat | Dog
    After call: Dog | Cat

 2. Passing two references:
    Before call: Dog | Tom
    Swapped in function: Tom | Dog
    After call: Tom | Dog

 3. Passing two references in variables:
    Before call: One | Two
    Before call: One | Two
    Swapped in function: Two | One
    After call: One | Two
    After call: One | Two

Do you see anything interesting in the output?

There is nothing strange in test 1. Pass-by-value on regular variables left no changes on original variables.

But in test 2, pass-by-value on references caused original variables to be swapped. I think I know why. The swap() function received copies of references, &$x and &$y, swapping references actually swapped the original variables, $x and $y.

But in test 3, pass-by-value on references stored in variables caused original variables un-touched. Initially, I thought test 3 should behave like test 2. But by following the pass-by-value logic, I think I know why it behaves differently. In the call, swap($x, $y), copies of values of $x and $y are passed. Since $x and $y are reference variables, values of $x and $y are values of $a and $b, "One" and "Two". So copies of "One" and "Two" are passed into the function. Of course, swapping these copies will have no impact on original values of $a and $b (or $x and $y, since they are aliases of $a and $b).

Conclusion, If you call a function with a reference expression as a pass-by-value argument, you are equivalently using this argument as a pass-by-reference argument. Changes on this argument inside the function will be applied to the original variable in the calling code.

If you run this tutorial example with PHP 5, you will get a fatal error:

C:\herong> \php-5.6\php PassByValueWithReference.php

PHP Fatal error:  Call-time pass-by-reference has been removed; 
If you would like to pass argument by reference, modify the declaration 
of swap() in C:\herong\PassByValueWithReference.php on line 20.

If you run this tutorial example with PHP 7, you will get a parsing error:

C:\herong> \php-7.3\php PassByValueWithReference.php
PHP Parse error:  syntax error, unexpected '&' 
in C:\herong\PassByValueWithReference.php on line 20

Last update: 2019.

Table of Contents

About This Book

Introduction and Installation of PHP 7.3

PHP Script File Syntax

PHP Data Types and Data Literals

Variables, References, and Constants

Expressions, Operations and Type Conversions

Conditional Statements - "if" and "switch"

Loop Statements - "while", "for", and "do ... while"

►Function Declaration, Arguments, and Return Values

What Is a Function