Luku 7. Variables

Sis�llys
Basics
Predefined variables
Variable scope
Variable variables
Variables from outside PHP

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]*'

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

$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
$t�yte = 'mansikka';    // valid; '�' is 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 Expressions.

PHP 4 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. This also means that no copying is performed; thus, the assignment happens more quickly. However, any speedup will likely be noticed only in tight loops or when assigning large arrays or objects.

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.
?>

If, for some reason you need to declare a dynamic variable global, say $usertype1, $usertype2 etc... do it like this: $i=1; $ready=#number of generated variables# while ($i < $ready) { $var="usertype$i"; global $$var; ###do the action with the var, for example:## echo $$var; $i++; }

I had trouble digging this out, and searches for various terms (memory model, garbage collection, &c) were not productive.

PHP4 uses reference counting for garbage collection. Details about this system can be found here:

a great way to pass variables from page to page... select: How PHP/FI handles GET and POST method data ie: /cgi-bin/php.cgi/[email protected]&var=value The relevant components of the PHP symbol table will be: $argc = 4 $argv[0] = abc $argv[1] = def $argv[2] = [email protected]&var=value $EMAIL_ADDR = [email protected] $var = value

Here is a tip to save the bandwidth rainforest. There is no reason to send meta tags to Mozilla compatible browsers.

<?php if(!strstr($HTTP_USER_AGENT, "Mozilla")) { ?> <meta name="author" content="I, me and myself" /> <? } ?>

Actually, you can use other chars (including spaces) in the variable name, if you are using variable variables. E.g., the following works for me (in PHP 4.1.1): $x="blah blah-blah"; $$x="value"; echo "var==" . $$x; echo "var==" . ${"blah blah-blah"}; Its output is: var==value var==value You can use it also in object variables (properties).

In the example listed above: <?php $foo = 'Bob'; // Assign the value 'Bob' to $foo $bar = &$foo; // Reference $foo via $bar. $bar = "My name is $bar"; // Alter $bar... echo $foo; // $foo is altered too. echo $bar; ?> the documentation above it is somewhat ambiguous: $bar = &$foo means that $foo becomes the reference for $bar, thus $foo = "Bob". As I read it, $bar should've become the reference for $foo, took me a bit to turn it around. Just some clarification.

Another way to assign a variable a large amount text without having to worry about quotes getting in the way is like so: $aVariable = <<<END "HEY!" END; print "$aVariable"; output: "HEY!" Where as this is will produce an error: $aVariable = ""HEY!"";

<<------------------< Env. Variables >------------------> If you are looking for an explanation of environment variables, go to If you wanted to set environment variables, look into the putenv() function.


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