Learning PHP--Variable variables,--Variable variables_PHP tutorial

PHP learning--Variable variables,--Variable variables

Variable variables

Sometimes it is convenient to use mutable variable names. That is to say, the variable name of a variable can be set and used dynamically. An ordinary variable is set by declaration, for example:

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

A variable variable obtains the value of an ordinary variable as the variable name of the variable variable. In the above example, hello uses two dollar signs ($), and it can be used as a variable variable. For example:

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

At this time, two variables are defined: the content of $a is "hello" and the content of $hello The content is "world". Therefore, the following statement:

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

The following statement outputs exactly the same result:

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

They will all output: hello world.

To use mutable variables with arrays, an ambiguity must be resolved. This is what the parser needs to know when writing $$a[1] to want $a[1] as As for a variable, we still want to use $$a as a variable and take out the value with index [1] in the variable. The syntax to solve this problem is to use ${$a[1]} for the first case and ${$a for the second case }[1].

Class properties can also be accessed through mutable property names. Mutable property names will be resolved within the scope of the call. For example, for the $foo->$bar expression, $bar will be parsed in the local scope and its value will be Used for the attribute name of $foo. The same is true for $bar when it is an array unit.

You can also use curly braces to clearly delimit attribute names. Most useful when the property is in an array, or the property name contains multiple parts or the property name contains illegal characters (such as from json_decode() or SimpleXML).

Example #1 Variable attribute example

<?<span>php
</span><span>class</span><span> foo {
    </span><span>var</span> <span>$bar</span> = 'I am bar.'<span>;
    </span><span>var</span> <span>$arr</span> = <span>array</span>('I am A.', 'I am B.', 'I am C.'<span>);
    </span><span>var</span> <span>$r</span>   = 'I am r.'<span>;
}

</span><span>$foo</span> = <span>new</span><span> foo();
</span><span>$bar</span> = 'bar'<span>;
</span><span>$baz</span> = <span>array</span>('foo', 'bar', 'baz', 'quux'<span>);
</span><span>echo</span> <span>$foo</span>-><span>$bar</span> . "\n"<span>;
</span><span>echo</span> <span>$foo</span>-><span>$baz</span>[1] . "\n"<span>;

</span><span>$start</span> = 'b'<span>;
</span><span>$end</span>   = 'ar'<span>;
</span><span>echo</span> <span>$foo</span>->{<span>$start</span> . <span>$end</span>} . "\n"<span>;

</span><span>$arr</span> = 'arr'<span>;
</span><span>echo</span> <span>$foo</span>-><span>$arr</span>[1] . "\n"<span>;
</span><span>echo</span> <span>$foo</span>->{<span>$arr</span>}[1] . "\n"<span>;

</span>?>

The above routine will output:

I am bar.<span>
I am bar</span>.<span>
I am bar</span>.<span>
I am r</span>.<span>
I am B</span>.

Example #2 Variable attribute example

<?<span>php

  </span><span>//</span><span>You can even add more Dollar Signs</span>

  <span>$Bar</span> = "a"<span>;
  </span><span>$Foo</span> = "Bar"<span>;
  </span><span>$World</span> = "Foo"<span>;
  </span><span>$Hello</span> = "World"<span>;
  </span><span>$a</span> = "Hello"<span>;

  </span><span>$a</span>; <span>//</span><span>Returns Hello</span>
  $<span>$a</span>; <span>//</span><span>Returns World</span>
  $$<span>$a</span>; <span>//</span><span>Returns Foo</span>
  $$$<span>$a</span>; <span>//</span><span>Returns Bar</span>
  $$$$<span>$a</span>; <span>//</span><span>Returns a</span>
<span>
  $$$$$</span><span>$a</span>; <span>//</span><span>Returns Hello</span>
  $$$$$$<span>$a</span>; <span>//</span><span>Returns World

  //... and so on ...//</span>

?>

Example #3 Variable attribute example

<?<span>php
</span><span>//</span><span> Given these variables ...</span>
<span>$nameTypes</span>    = <span>array</span>("first", "last", "company"<span>);
</span><span>$name_first</span>   = "John"<span>;
</span><span>$name_last</span>    = "Doe"<span>;
</span><span>$name_company</span> = "PHP.net"<span>;

</span><span>//</span><span> Then this loop is ...</span>
<span>foreach</span>(<span>$nameTypes</span> <span>as</span> <span>$type</span><span>)
  </span><span>print</span> ${"name_<span>$type</span>"} . "\n"<span>;

</span><span>//</span><span> ... equivalent to this print statement.</span>
<span>print</span> "<span>$name_first</span>\n<span>$name_last</span>\n<span>$name_company</span>\n"<span>;
</span>?>

The above routine will output:

<span>John
Doe
PHP</span>.<span>net
John
Doe
PHP</span>.net

Excerpted from: http://php.net/manual/zh/functions.variable-functions.php

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