10 PHP constants

Constants are similar to variables, but once a constant is defined, it cannot be changed or undefined.

PHP Constants

Constants are identifiers (names) of a single value. This value cannot be changed in script.

Valid constant names start with a character or an underscore (no $ sign in front of the constant name).

Note: Unlike variables, constants are automatically global throughout the entire script.

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Set PHP constants

To set a constant, use the define() function - it takes three parameters:

<code>首个参数定义常量的名称
第二个参数定义常量的值
可选的第三个参数规定常量名是否对大小写敏感。默认是 false，即敏感。
</code>

The following example creates a case-sensitive constant with the value "Welcome to W3School.com. cn!":

<code><span><span><?php</span><span>// 定义对大小写敏感的常量</span>
define(<span>"GREETING"</span>, <span>"Welcome to W3School.com.cn!"</span>);
<span>echo</span> GREETING;
<span>echo</span><span>"<br>"</span>;
<span>// 不会输出常量的值</span><span>echo</span> greeting;
<span>?></span></span></code>

The following example creates a case-insensitive constant with the value "Welcome to W3School.com.cn!":

<code><span><span><span><?php</span><span>// 定义对大小写不敏感的常量</span>
define(<span>"GREETING"</span>, <span>"Welcome to W3School.com.cn!"</span>, <span>true</span>);
<span>echo</span> GREETING;
<span>echo</span><span>"<br>"</span>;
<span>// 会输出常量的值</span><span>echo</span> greeting;
<span>?></span></span></span></code>

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