Constants are similar to the variable except once they defined, they can never be undefined or changed. A constant value cannot change during the execution of the script. Once a constant is defined, it cannot be undefined or redefined.
Generally, PHP constants are defined in uppercase letters.
PHP constants follow the same PHP variable naming rules. A valid constant name starts with a letter or underscore (no $ sign before the constant name). Constants are case-sensitive.
Unlike variables, constants are automatically global across the entire script.
The define() function in PHP is used to create a constant.
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PHP constant: define()
Use the define() function to create a constant. It defines constant at run time.
Syntax
define(ConstantName, value, case-insensitive)- ConstantName: It specifies the constant name.
- value: The value to be stored in the constant.
- case-insensitive: Specifies whether a constant is case-insensitive. Default value is false. It means it is case sensitive by default.
Example
<?php
define("MESSAGE","Hello The New Tutorial PHP<br>");
echo MESSAGE;
//Create a constant with case-insensitive name:
define("MESSAGE1","Hello Hello The New Tutorial.com PHP",true);//not case sensitive
echo MESSAGE1, "</br>";
echo message1;
?>