玩转php常量
常量的命名规则
命名: 与变量相同的命名规则
方法:define()函数
格式:define('常量名','具体值');
例子:define('PI',3.14);
常量与变量的比较
相同:
1:命名规则相同(但一般为大写)
不同:
1:引用常量时不用$,直接用名称.
2:常量只能用define定义,不能用赋值语句
3:常量一旦定义就不能重新定义或取消定义
4:常量的值只能是标量(只能是整型、浮点型、布尔型、字符型、NULL)
<?php //定义常量 define('pi',3.14); //计算园的面的面积 $r=3; echo pi*$r*$r; //结果:28.26 //常量定义过后,不能再重新定义 define('pi',3.23); //结果:Notice: Constant pi already defined in C:\wamp\www\0124\05.php on line 15 //常量不能重新赋值 pi=100; echo pi; //结果:Parse error: syntax error, unexpected '=' in C:\wamp\www\0124\05.php on line 21 //会有语法上的错误 $PI=10; function text(){ echo $PI;//函数有作用域。此$PI变量是函数里面的与外边的$PI不是一个。 echo pi;//常量一次定义,可以再任何地方使用 } //结果:Notice: Undefined variable: PI in C:\wamp\www\0124\05.php on line 31 text(); ?>
如何判断一个常量有没有定义?
define 定义常量
defined 判断常量
<?php if(defined('PI')){ echo 'PI常量已经定义了。<br/>'; }else{ echo 'PI常量未定义,我来定义它<br/>'; define('PI',33322); } echo PI; ?>
结果:PI常量未定义,我来定义它 33322
动态使用常量方法
<?php //访问:http://localhost/0124/06.php?who=DA $who=$_GET['who']; //动态根据传来的值,显示不同的常量 define('DA','刘备'); define('ER','关羽'); define('SAN','张飞'); echo $who,'<br/>'; //以$who=DA为例 //$who变量的值为DA,解析变量得到DA,打印DA,是没错 //但问题是,我们想解析出$who的值,即DA,再把DA当成常量的名,再解析出常量的值来 //想用动态常量的话,即想用变量的值当成常量名的话,必须要考constant 函数才可以。 echo constant($who); //结果:DA // 刘备 ?>

What’s still popular is the ease of use, flexibility and a strong ecosystem. 1) Ease of use and simple syntax make it the first choice for beginners. 2) Closely integrated with web development, excellent interaction with HTTP requests and database. 3) The huge ecosystem provides a wealth of tools and libraries. 4) Active community and open source nature adapts them to new needs and technology trends.

PHP and Python are both high-level programming languages that are widely used in web development, data processing and automation tasks. 1.PHP is often used to build dynamic websites and content management systems, while Python is often used to build web frameworks and data science. 2.PHP uses echo to output content, Python uses print. 3. Both support object-oriented programming, but the syntax and keywords are different. 4. PHP supports weak type conversion, while Python is more stringent. 5. PHP performance optimization includes using OPcache and asynchronous programming, while Python uses cProfile and asynchronous programming.

PHP is mainly procedural programming, but also supports object-oriented programming (OOP); Python supports a variety of paradigms, including OOP, functional and procedural programming. PHP is suitable for web development, and Python is suitable for a variety of applications such as data analysis and machine learning.

PHP originated in 1994 and was developed by RasmusLerdorf. It was originally used to track website visitors and gradually evolved into a server-side scripting language and was widely used in web development. Python was developed by Guidovan Rossum in the late 1980s and was first released in 1991. It emphasizes code readability and simplicity, and is suitable for scientific computing, data analysis and other fields.

PHP is suitable for web development and rapid prototyping, and Python is suitable for data science and machine learning. 1.PHP is used for dynamic web development, with simple syntax and suitable for rapid development. 2. Python has concise syntax, is suitable for multiple fields, and has a strong library ecosystem.

PHP remains important in the modernization process because it supports a large number of websites and applications and adapts to development needs through frameworks. 1.PHP7 improves performance and introduces new features. 2. Modern frameworks such as Laravel, Symfony and CodeIgniter simplify development and improve code quality. 3. Performance optimization and best practices further improve application efficiency.

PHPhassignificantlyimpactedwebdevelopmentandextendsbeyondit.1)ItpowersmajorplatformslikeWordPressandexcelsindatabaseinteractions.2)PHP'sadaptabilityallowsittoscaleforlargeapplicationsusingframeworkslikeLaravel.3)Beyondweb,PHPisusedincommand-linescrip

PHP type prompts to improve code quality and readability. 1) Scalar type tips: Since PHP7.0, basic data types are allowed to be specified in function parameters, such as int, float, etc. 2) Return type prompt: Ensure the consistency of the function return value type. 3) Union type prompt: Since PHP8.0, multiple types are allowed to be specified in function parameters or return values. 4) Nullable type prompt: Allows to include null values and handle functions that may return null values.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

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

Hot Article

Hot Tools

MantisBT
Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

MinGW - Minimalist GNU for Windows
This project is in the process of being migrated to osdn.net/projects/mingw, you can continue to follow us there. MinGW: A native Windows port of the GNU Compiler Collection (GCC), freely distributable import libraries and header files for building native Windows applications; includes extensions to the MSVC runtime to support C99 functionality. All MinGW software can run on 64-bit Windows platforms.

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool

VSCode Windows 64-bit Download
A free and powerful IDE editor launched by Microsoft