home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

Programming PHPProgramming PHPSearch this book

2.3. Variables

Variables in PHP are identifiers prefixed with a dollar sign ($). For example:


A variable may hold a value of any type. There is no compile- or runtime type checking on variables. You can replace a variable's value with another of a different type:

$what = "Fred";
$what = 35;
$what = array('Fred', '35', 'Wilma');

There is no explicit syntax for declaring variables in PHP. The first time the value of a variable is set, the variable is created. In other words, setting a variable functions as a declaration. For example, this is a valid complete PHP program:

$day = 60 * 60 * 24;
echo "There are $day seconds in a day.\n";
There are 86400 seconds in a day.

A variable whose value has not been set behaves like the NULL value:

if ($uninitialized_variable === NULL) {
  echo "Yes!";

2.3.3. Variable Scope

The scope of a variable, which is controlled by the location of the variable's declaration, determines those parts of the program that can access it. There are four types of variable scope in PHP: local, global, static, and function parameters. Function parameters

As we'll discuss in more detail in Chapter 3, a function definition can have named parameters:

function greet ($name) {
  echo "Hello, $name\n";
Hello, Janet

Function parameters are local, meaning that they are available only inside their functions. In this case, $name is inaccessible from outside greet( ).

2.3.4. Garbage Collection

PHP uses reference counting and copy-on-write to manage memory. Copy-on-write ensures that memory isn't wasted when you copy values between variables, and reference counting ensures that memory is returned to the operating system when it is no longer needed.

To understand memory management in PHP, you must first understand the idea of a symbol table . There are two parts to a variable—its name (e.g., $name), and its value (e.g., "Fred"). A symbol table is an array that maps variable names to the positions of their values in memory.

When you copy a value from one variable to another, PHP doesn't get more memory for a copy of the value. Instead, it updates the symbol table to say "both of these variables are names for the same chunk of memory." So the following code doesn't actually create a new array:

$worker = array("Fred", 35, "Wilma");
$other = $worker;                        // array isn't copied

If you then modify either copy, PHP allocates the memory and makes the copy:

$worker[1] = 36;                         // array is copied, value changed

By delaying the allocation and copying, PHP saves time and memory in a lot of situations. This is copy-on-write.

Each value pointed to by a symbol table has a reference count, a number that represents the number of ways there are to get to that piece of memory. After the initial assignment of the array to $worker and $worker to $other, the array pointed to by the symbol table entries for $worker and $other has a reference count of 2.[1] In other words, that memory can be reached two ways: through $worker or $other. But after $worker[1] is changed, PHP creates a new array for $worker, and the reference count of each of the arrays is only 1.

[1]It is actually 3 if you are looking at the reference count from the C API, but for the purposes of this explanation and from a user-space perspective, it is easier to think of it as 2.

When a variable goes out of scope (as a function parameter or local variable does at the end of a function), the reference count of its value is decreased by one. When a variable is assigned a value in a different area of memory, the reference count of the old value is decreased by one. When the reference count of a value reaches 0, its memory is freed. This is reference counting.

Reference counting is the preferred way to manage memory. Keep variables local to functions, pass in values that the functions need to work on, and let reference counting take care of freeing memory when it's no longer needed. If you do insist on trying to get a little more information or control over freeing a variable's value, use the isset( ) and unset( ) functions.

To see if a variable has been set to something, even the empty string, use isset( ):

$s1 = isset($name);                    // $s1 is false
$name = "Fred";
$s2 = isset($name);                    // $s2 is true

Use unset( ) to remove a variable's value:

$name = "Fred";
unset($name);                          // $name is NULL

Library Navigation Links

Copyright © 2003 O'Reilly & Associates. All rights reserved.