IMPRESS dotted_line CONTACT dotted_line search dotted_line Kontakt dotted_line sitemap dotted_line print
194 PHP-Resource users online

Switch to another languags Deutsch aktuelle Sprache Englisch
ebiz-consult PHP Entwicklung
- Ad -
php-resource

PHP Manual

Variables

Variables

Table of Contents

Basics

Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.

Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'

Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255 (0x7f-0xff).

Note: $this is a special variable that can't be assigned.

Tip

You may also want to take a look at the Userland Naming Guide.

For information on variable related functions, see the Variable Functions Reference.

<?php
$var = 'Bob';
$Var = 'Joe';
echo "$var, $Var";      // outputs "Bob, Joe"

$4site = 'not yet';     // invalid; starts with a number
$_4site = 'not yet';    // valid; starts with an underscore
$täyte = 'mansikka';    // valid; 'ä' is (Extended) ASCII 228.
?>

In PHP 3, variables are always assigned by value. That is to say, when you assign an expression to a variable, the entire value of the original expression is copied into the destination variable. This means, for instance, that after assigning one variable's value to another, changing one of those variables will have no effect on the other. For more information on this kind of assignment, see the chapter on Expressions.

As of PHP 4, PHP offers another way to assign values to variables: assign by reference. This means that the new variable simply references (in other words, "becomes an alias for" or "points to") the original variable. Changes to the new variable affect the original, and vice versa.

To assign by reference, simply prepend an ampersand (&) to the beginning of the variable which is being assigned (the source variable). For instance, the following code snippet outputs 'My name is Bob' twice:

<?php
$foo = 'Bob';              // Assign the value 'Bob' to $foo
$bar = &$foo;              // Reference $foo via $bar.
$bar = "My name is $bar";  // Alter $bar...
echo $bar;
echo $foo;                 // $foo is altered too.
?>

One important thing to note is that only named variables may be assigned by reference.

<?php
$foo = 25;
$bar = &$foo;      // This is a valid assignment.
$bar = &(24 * 7);  // Invalid; references an unnamed expression.

function test()
{
   return 25;
}

$bar = &test();    // Invalid.
?>

It is not necessary to initialize variables in PHP however it is a very good practice. Uninitialized variables have a default value of their type - FALSE, zero, empty string or an empty array.

Example#1 Default values of uninitialized variables

<?php
echo ($unset_bool ? "true" : "false"); // false
$unset_int += 25; // 0 + 25 => 25
echo $unset_string . "abc"; // "" . "abc" => "abc"
$unset_array[3] = "def"; // array() + array(3 => "def") => array(3 => "def")
?>

Relying on the default value of an uninitialized variable is problematic in the case of including one file into another which uses the same variable name. It is also a major security risk with register_globals turned on. E_NOTICE level error is issued in case of working with uninitialized variables, however not in the case of appending elements to the uninitialized array. isset() language construct can be used to detect if a variable has been already initialized.



Comments to the PHP manual
Write new comment
 

 
PHP-Index
  1. Introduction
  2. Installation and Configuration
  3. Language reference
  4. Security
  5. Features
  6. Function reference
  7. PHP and zend engine internal
  8. FAQ: Frequently Asked Questions
  9. Appendices



 

New Tutorial entries

plotting masters - a professional guide - Teil I

Grafische Interpolation und Bestapproximation von numerischen Wertepaaren. Mit anderen Worten: Wir wollen Punkte auf einer Zeichenebene über verschiedene Verfahren miteinander verbinden.

EVAMasters | Category: PHP
plotting masters - a professional guide - Teil II

Grafische Interpolation und Bestapproximation von numerischen Wertepaaren. Mit anderen Worten: Wir wollen Punkte auf einer Zeichenebene über verschiedene Verfahren miteinander verbinden.

EVAMasters | Category: PHP
Spielereien mit Zeit und Datum

Das Rechnen mit Datum und Zeit ist nur selten unproblematisch, PHP stellt uns dafür zahlreiche Funktionen zur Verfügung. Wir wollen uns im folgenden ein paar davon ansehen und an Beispielen erproben.

Stephane | Category: PHP
Eines Warenkorbsystem für Anfänger

Ein kurzes Tutorial für Anfänger um ein einfaches aber funktionelles Warenkorbsystem zu erstellen. Freue mich über Rückmeldungen und Kritik - denn nur so verbessert man sich :)

web3hoster | Category: PHP
SVN Server einrichten

Wie man unter Linux einen Subversion Server einrichtet.

jens | Category: LINUX
Programmierung - SQL

Dieser SQL-Kurs wendet sich vor allem an Programmierer, die mit Hilfe von SQL auf Datebaken zugreifen, und an alle, die Datenbanken neu entwerfen, erweitern oder abändern wollen

webmaster205@ | Category: MySQL