Variable Variable Name - Name Variables with Expressions

This section describes what is a variable variable name, storing the variable name in another variable, naming variables with expressions, using any expressions as variable names.

What is a Variable Variable Name: A variable variable name is a variable name that is stored inside another variable. Normally, a variable name is a static string. But sometimes, you want to store a variable name inside another variable to be able to participate in operations. PHP supports variable variable names with following basic rules:

1. If the name of the original variable is stored in a different variable, you can use the $$var notation to identify the original variable in your PHP source code. For example, $variableName = "title"; $$variableName represents the variable $title.

2. If the name of the original variable can be calculated by an expression, you can use the ${exp} notation to identify the original variable in your PHP source code. For example, $variableName = "title"; ${$variableName."_1"} represents the variable $title_1.

3. Variables with variable names can be assigned with new data like normal variables. For example, $variableName = "title"; $$variableName = "Herong's PHP Book"; assigns "Herong's PHP Book" to $title.

4. Variables with variable names can be used in any operations like normal variables. $variableName = "title"; $$variableName = "Herong's PHP Book"; print($$variableName." is free"); prints "Herong's PHP Book is free".

5. If you use an expression as the variable name, the result of the expression will be automatically converted to a string, which will be used as the variable name. With this automatic conversion, anything can be used as a variable name. Even an empty string can be used as a variable name, ${""} is a perfect variable expression.

Now let's look at my tutorial example below to confirm some rules mentioned above:

<?php # VariableVariableName.php
#- Copyright (c) 2015, HerongYang.com, All Rights Reserved.
#
   $variableName = "title"; 
   $$variableName = "Herong's PHP Book";
   print "\n \$\$variableName is assigned to a message:\n";
   print "    \$variableName: "; var_dump($variableName);
   print "    \$\$variableName: "; var_dump($$variableName);
   print "    \$title: "; var_dump($title);

   $variableName = ""; 
   $$variableName = "Herong's Programming Book";
   print "\n \$variableName contains an empty string:\n";
   print "    \$variableName: "; var_dump($variableName);
   print "    \$\$variableName: "; var_dump($$variableName);
   print "    \${''}: "; var_dump(${''});

   $variableName = NULL; 
   $$variableName = "Herong's Tutorial Book";
   print "\n \$variableName contains the NULL:\n";
   print "    \$variableName: "; var_dump($variableName);
   print "    \$\$variableName: "; var_dump($$variableName);
   print "    \${NULL}: "; var_dump(${NULL});

   $variableName = 1.0/3.3; 
   $$variableName = "Herong's Free Book";
   print "\n \$variableName contains a float number:\n";
   print "    \$variableName: "; var_dump($variableName);
   print "    \$\$variableName: "; var_dump($$variableName);
   print "    \${1.0/3.3}: "; var_dump(${1.0/3.3});
?>

If you run this sample script, you should get:

 $$variableName is assigned to a message:
    $variableName: string(5) "title"
    $$variableName: string(17) "Herong's PHP Book"
    $title: string(17) "Herong's PHP Book"

 $variableName contains an empty string:
    $variableName: string(0) ""
    $$variableName: string(25) "Herong's Programming Book"
    ${''}: string(25) "Herong's Programming Book"

 $variableName contains the NULL:
    $variableName: NULL
    $$variableName: string(22) "Herong's Tutorial Book"
    ${NULL}: string(22) "Herong's Tutorial Book"

 $variableName contains a float number:
    $variableName: float(0.30303030303)
    $$variableName: string(18) "Herong's Free Book"
    ${1.0/3.3}: string(18) "Herong's Free Book"

Last update: 2015.

Table of Contents

 About This Book

 Introduction and Installation of PHP 7.0

 PHP Script File Syntax

 PHP Data Types and Data Literals

Variables, References, and Constants

 Variables and Assignment Operations

 References and Variables

Variable Variable Name - Name Variables with Expressions

 Constant and define() Function

 Expressions, Operations and Type Conversions

 Conditional Statements - "if" and "switch"

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

 Function Declaration, Arguments, and Return Values

 Arrays - Ordered Maps

 Configuring and Sending out Emails

 Retrieving Information from HTTP Requests

 Creating and Managing Sessions in PHP Scripts

 Sending and Receiving Cookies in PHP Scripts

 Controlling HTTP Response Header Lines in PHP Scripts

 MySQL Server Connection and Access Functions

 Functions to Manage Directories, Files and Images

 SOAP Extension Function and Calling Web Services

 SOAP Server Functions and Examples

 Localization Overview of Web Applications

 Using Non-ASCII Characters in HTML Documents

 Using Non-ASCII Characters as PHP Script String Literals

 Receiving Non-ASCII Characters from Input Forms

 "mbstring" Extension and Non-ASCII Encoding Management

 Managing Non-ASCII Character Strings with MySQL Servers

 Introduction of Class and Object

 Integrating PHP with Apache Web Server

 Outdated Tutorials

 References

 PDF Printing Version