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:
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.
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:
produces the exact same output as:
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.
Class properties may also be accessed using variable property names. The
variable property name will be resolved within the scope from which the
call is made. For instance, if you have an expression such as
$foo->$bar, then the local scope will be examined for
$bar and its value will be used as the name of the
property of $foo. This is also true if
$bar is an array access.
Curly braces may also be used, to clearly delimit the property
name. They are most useful when accessing values within a property that
contains an array, when the property name is made of multiple parts,
or when the property name contains characters that are not
otherwise valid (e.g. from json_decode()
or SimpleXML).
Example #1 Variable property example
<?php
class foo {
var $bar = 'I am bar.';
var $arr = array('I am A.', 'I am B.', 'I am C.');
var $r = 'I am r.';
}
$foo = new foo();
$bar = 'bar';
$baz = array('foo', 'bar', 'baz', 'quux');
echo $foo->$bar . "\n";
echo $foo->{$baz[1]} . "\n";
$start = 'b';
$end = 'ar';
echo $foo->{$start . $end} . "\n";
$arr = 'arr';
echo $foo->{$arr[1]} . "\n";
?>
Il precedente esempio visualizzerà:
I am bar.
I am bar.
I am bar.
I am r.
Avviso
Please note that variable variables cannot be used with PHP's
Superglobal arrays
within functions or class methods. The variable $this
is also a special variable that cannot be referenced dynamically.