Manuale PHP
Indietro	Capitolo 12. Variables	Avanti

Variable variables

Sometimes it is convenient to be able to have variable variable names. That is, a variable name which can be set and used dynamically. A normal variable is set with a statement such as:


<?php
$a = 'hello';
?>

A variable variable takes the value of a variable and treats that as the name of a variable. In the above example, hello, can be used as the name of a variable by using two dollar signs. i.e.


<?php
$$a = 'world';
?>

At this point two variables have been defined and stored in the PHP symbol tree: $a with contents "hello" and $hello with contents "world". Therefore, this statement:


<?php
echo "$a ${$a}";
?>

produces the exact same output as:


<?php
echo "$a $hello";
?>

i.e. they both produce: hello world.

In order to use variable variables with arrays, you have to resolve an ambiguity problem. That is, if you write $$a[1] then the parser needs to know if you meant to use $a[1] as a variable, or if you wanted $$a as the variable and then the [1] index from that variable. The syntax for resolving this ambiguity is: ${$a[1]} for the first case and ${$a}[1] for the second.

Avvertimento

Please note that variable variables cannot be used with PHP's Superglobal arrays within functions or class methods.

Indietro	Partenza	Avanti
Variable scope	Risali	Variables from outside PHP