Table of Contents
PHP的学习--可变变量,--可变变量
可变变量
Home php教程 php手册 PHP的学习--可变变量,--可变变量

PHP的学习--可变变量,--可变变量

Jun 13, 2016 am 08:50 AM
php

PHP的学习--可变变量,--可变变量

可变变量

有时候使用可变变量名是很方便的。就是说,一个变量的变量名可以动态的设置和使用。一个普通的变量通过声明来设置,例如:

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

一个可变变量获取了一个普通变量的值作为这个可变变量的变量名。在上面的例子中 hello 使用了两个美元符号($)以后,就可以作为一个可变变量的变量了。例如:

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

这时,两个变量都被定义了:$a 的内容是“hello”并且 $hello 的内容是“world”。因此,以下语句:

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

与以下语句输出完全相同的结果:

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

它们都会输出:hello world。

要将可变变量用于数组,必须解决一个模棱两可的问题。这就是当写下 $$a[1] 时,解析器需要知道是想要 $a[1] 作为一个变量呢,还是想要$$a 作为一个变量并取出该变量中索引为 [1] 的值。解决此问题的语法是,对第一种情况用 ${$a[1]},对第二种情况用 ${$a}[1]

类的属性也可以通过可变属性名来访问。可变属性名将在该调用所处的范围内被解析。例如,对于 $foo->$bar 表达式,则会在本地范围来解析$bar 并且其值将被用于 $foo 的属性名。对于 $bar 是数组单元时也是一样。

也可使用花括号来给属性名清晰定界。最有用是在属性位于数组中,或者属性名包含有多个部分或者属性名包含有非法字符时(例如来自json_decode() 或 SimpleXML)。

Example #1 可变属性示例

<?<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>?>
Copy after login

以上例程会输出:

I am bar.<span>
I am bar</span>.<span>
I am bar</span>.<span>
I am r</span>.<span>
I am B</span>.
Copy after login

Example #2 可变属性示例

<?<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>

?>
Copy after login

Example #3 可变属性示例

<?<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>?>
Copy after login

以上例程会输出:

<span>John
Doe
PHP</span>.<span>net
John
Doe
PHP</span>.net
Copy after login

 

摘自:http://php.net/manual/zh/functions.variable-functions.php

 

Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

PHP 8.4 Installation and Upgrade guide for Ubuntu and Debian PHP 8.4 Installation and Upgrade guide for Ubuntu and Debian Dec 24, 2024 pm 04:42 PM

PHP 8.4 brings several new features, security improvements, and performance improvements with healthy amounts of feature deprecations and removals. This guide explains how to install PHP 8.4 or upgrade to PHP 8.4 on Ubuntu, Debian, or their derivati

Explain JSON Web Tokens (JWT) and their use case in PHP APIs. Explain JSON Web Tokens (JWT) and their use case in PHP APIs. Apr 05, 2025 am 12:04 AM

JWT is an open standard based on JSON, used to securely transmit information between parties, mainly for identity authentication and information exchange. 1. JWT consists of three parts: Header, Payload and Signature. 2. The working principle of JWT includes three steps: generating JWT, verifying JWT and parsing Payload. 3. When using JWT for authentication in PHP, JWT can be generated and verified, and user role and permission information can be included in advanced usage. 4. Common errors include signature verification failure, token expiration, and payload oversized. Debugging skills include using debugging tools and logging. 5. Performance optimization and best practices include using appropriate signature algorithms, setting validity periods reasonably,

How do you parse and process HTML/XML in PHP? How do you parse and process HTML/XML in PHP? Feb 07, 2025 am 11:57 AM

This tutorial demonstrates how to efficiently process XML documents using PHP. XML (eXtensible Markup Language) is a versatile text-based markup language designed for both human readability and machine parsing. It's commonly used for data storage an

Explain late static binding in PHP (static::). Explain late static binding in PHP (static::). Apr 03, 2025 am 12:04 AM

Static binding (static::) implements late static binding (LSB) in PHP, allowing calling classes to be referenced in static contexts rather than defining classes. 1) The parsing process is performed at runtime, 2) Look up the call class in the inheritance relationship, 3) It may bring performance overhead.

PHP Program to Count Vowels in a String PHP Program to Count Vowels in a String Feb 07, 2025 pm 12:12 PM

A string is a sequence of characters, including letters, numbers, and symbols. This tutorial will learn how to calculate the number of vowels in a given string in PHP using different methods. The vowels in English are a, e, i, o, u, and they can be uppercase or lowercase. What is a vowel? Vowels are alphabetic characters that represent a specific pronunciation. There are five vowels in English, including uppercase and lowercase: a, e, i, o, u Example 1 Input: String = "Tutorialspoint" Output: 6 explain The vowels in the string "Tutorialspoint" are u, o, i, a, o, i. There are 6 yuan in total

What are PHP magic methods (__construct, __destruct, __call, __get, __set, etc.) and provide use cases? What are PHP magic methods (__construct, __destruct, __call, __get, __set, etc.) and provide use cases? Apr 03, 2025 am 12:03 AM

What are the magic methods of PHP? PHP's magic methods include: 1.\_\_construct, used to initialize objects; 2.\_\_destruct, used to clean up resources; 3.\_\_call, handle non-existent method calls; 4.\_\_get, implement dynamic attribute access; 5.\_\_set, implement dynamic attribute settings. These methods are automatically called in certain situations, improving code flexibility and efficiency.

PHP and Python: Comparing Two Popular Programming Languages PHP and Python: Comparing Two Popular Programming Languages Apr 14, 2025 am 12:13 AM

PHP and Python each have their own advantages, and choose according to project requirements. 1.PHP is suitable for web development, especially for rapid development and maintenance of websites. 2. Python is suitable for data science, machine learning and artificial intelligence, with concise syntax and suitable for beginners.

PHP: A Key Language for Web Development PHP: A Key Language for Web Development Apr 13, 2025 am 12:08 AM

PHP is a scripting language widely used on the server side, especially suitable for web development. 1.PHP can embed HTML, process HTTP requests and responses, and supports a variety of databases. 2.PHP is used to generate dynamic web content, process form data, access databases, etc., with strong community support and open source resources. 3. PHP is an interpreted language, and the execution process includes lexical analysis, grammatical analysis, compilation and execution. 4.PHP can be combined with MySQL for advanced applications such as user registration systems. 5. When debugging PHP, you can use functions such as error_reporting() and var_dump(). 6. Optimize PHP code to use caching mechanisms, optimize database queries and use built-in functions. 7

See all articles