Example of Passing Arguments by References

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This chapter provides tutorial examples and notes about user-defined functions. Topics include defining function with arguments, calling functions in expressions, passing arguments by values or by references, variable-length argument list, providing default values for arguments, returning data by value or by reference.

As we learned from the previous section, PHP supports pass-by-reference arguments if they are defined with a & prefix on the argument variables. If an argument is defined as an pass-by-reference argument, the calling operation can provide a variable, for this argument. The reference of that variable will be taken and assigned to this argument variable when function is called. For example, if we have "function f(&$var) {...}" defined, "f($arg)" is equivalent to "{$var=&$arg; ...}".

To see how passing arguments by references works, I wrote this tutorial example, PassByReference.php:

<?php # PassByReference.php
# Copyright (c) 2003 by Dr. Herong Yang. http://www.herongyang.com/
# 
function swap(&$left, &$right) {
   $temp = $left;
   $left = $right; 
   $right = $temp;
   print("    Swapped in function: ".getString($left, $right)."\n");
}

#   print("\n 1. Passing two literals:\n");
#   print("    Before call: ". getString("Apple", "Orange") ."\n");
#   swap("Apple", "Orange");

   print("\n 2. 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 3. Passing two arrays:\n");
   $x = array("Mon", "Tue"); $y = array("Jan", "Feb");
   print("    Before call: ". getString($x, $y) ."\n");
   swap($x, $y);
   print("    After call: ". getString($x, $y) ."\n");

   print("\n 4. Passing two objects:\n");
   $x = new DateTime("2005-01-01"); $y = new DateTime("2006-01-01");
   print("    Before call: ". getString($x, $y) ."\n");
   swap($x, $y);
   print("    After call: ". getString($x, $y) ."\n");

function getString($left, $right) {
   if (is_scalar($left)) {
      return "$left | $right";
   } elseif (is_array($left)) {
      return "($left[0], $left[1]...) | ($right[0], $right[1]...)";
   } elseif (is_object($left) && get_class($left)=="DateTime") {
      return $left->format("Y")." | ".$right->format("Y");
   } else {
      return NULL;
   }
}
?>

If you run this sample script, you should get:

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

 3. Passing two arrays:
    Before call: (Mon, Tue...) | (Jan, Feb...)
    Swapped in function: (Jan, Feb...) | (Mon, Tue...)
    After call: (Jan, Feb...) | (Mon, Tue...)

 4. Passing two objects:
    Before call: 2005 | 2006
    Swapped in function: 2006 | 2005
    After call: 2006 | 2005

The output confirms that:

• My swap() function is very powerful. It swapped references of any types of variables received from the argument list.
• Pass-by-reference arguments can be used for scalar, array and object variables.
• Pass-by-reference arguments allow functions to reassign data values to original variables in the calling code.
• Pass-by-reference arguments can only take variables from the calling code. If you open test #1 in my sample script, the execution will fail with a message, "Fatal error: Only variables can be passed by reference".

Exercise: Try to test pass-by-reference arguments with:

   swap("Apple", "Orange");
   swap(array("Mon", "Tue"), array("Jan", "Feb"));
   swap(new DateTime("2005-01-01"), new DateTime("2006-01-01"));
   swap(&$a, &$b);

Last update: 2005.

Sections in This Chapter

What Is a Function?

"function" Statements - Defining Functions

Function Call Operations

Passing Arguments to Functions

Example of Passing Arguments by Values

Using Pass-by-Value Arguments for References

Example of Passing Arguments by References

Variable-Length Argument Lists

Providing Default Values to Argument Variables

Returning Values from Functions

Returning References from Functions