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This article mainly explains "what is const in php". Interested friends may wish to have a look at it. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what is const in php"?

Const in php is a keyword used to define a constant; when you use the const keyword to define a constant, you can only contain scalar data; you can define a constant as an expression or as an array.

The operating environment of this paper: Windows7 system, PHP7.1, Dell G3.

What does const mean in php?

Const in php is a keyword used to define a constant.

You can define a constant using either the const keyword or the define () function. The function define () allows you to define a constant as an expression, while the const keyword has some limitations, as shown in the following section. Once a constant is defined, it can no longer be changed or undefined.

When using the const keyword to define constants, you can only contain scalar data (bool, int, float, string). You can define a constant as either an expression or an array. Resource can also be defined as a constant, but should be avoided as much as possible, as it can cause unpredictable results.

You can simply get the value of a constant by specifying its name, and unlike variables, you should not precede a constant with a $sign. If the constant name is dynamic, you can also use the function constant () to get the value of the constant. You can get a list of all the defined constants with get_defined_constants ().

Note: constants and (global) variables are in different namespaces. This means that, for example, true and $TRUE are different.

If an undefined constant is used, an Error is thrown. Prior to PHP 8.0.0, calls to undefined constants were interpreted as a string of the constant, that is, (CONSTANT corresponds to "CONSTANT"). This method has been deprecated in PHP 7.2.0 and an E_WARNING level error is thrown. (an E_NOTICE-level error was issued before PHP 7.2.0. See why $Foo [bar] is wrong in the manual (unless bar is a constant). This does not apply to (fully) qualified constants and, if not defined, will always throw an Error.

Note: if you want to check whether a constant is defined, use the defined () function.

Constants and variables are different as follows:

The constant is not preceded by a dollar sign ($)

Constants can be defined and accessed anywhere regardless of the scope of the variable

Once a constant is defined, it cannot be redefined or undefined

Constants can only calculate scalar values or arrays.

Note:

In contrast to using define () to define constants, using the const keyword to define constants must be in the topmost scope, because this method is defined at compile time. This means that you cannot use const to define constants within functions, loops, or if or try/catch statements.

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