Detailed explanation of defining PHP constants_PHP tutorial

Constant can be understood as a variable whose value does not change. Once a constant value is defined, it cannot be changed anywhere else in the script. A constant consists of English letters, underscores, and numbers, but numbers cannot appear as the first letter.
Use the define() function in php to define constants. The syntax format of this function is:
define(string constant_name, mixed value, case_sensitive = true)

This function has 3 parameters:
constant_name: required parameter, constant name, that is, identifier
value: required parameter, constant value
case_sensitive: Optional parameter, specifies whether to be case sensitive, set to true to indicate insensitivity

There are two ways to obtain a constant value:
1. Use the constant name to directly obtain the value;
2. Use the constant() function.

The constant() function has the same effect as directly using the constant name to output, but the function can dynamically output different constants, so it must be flexible and convenient to use.

The syntax format is:

mixed constant(string constant_name)

The parameter constant_name is the name of the constant to be obtained, or it can be a variable that stores the constant name.

If successful, the value of the constant will be returned. If failed, an error message will be displayed. The constant is not defined.

To determine whether a constant has been defined, use the defined() function. The syntax format of the function is:

bool defined(string constants_name)

constant_name is the name of the constant to be obtained. If it exists, it returns true, otherwise it returns false;

You can use predefined constants in php to obtain information in php. Such as "_FILE_", "_LINE_", "PHP_OS", etc.

Example:

Copy code The code is as follows:

define ("MESSAGE", "PHP constant definition, constant names are case-sensitive");
echo MESSAGE."
"; //Output constant MESSAGE
echo Message."
"; //Output "Message", indicating that there is no such constant

define("MESSAGE2", "PHP constant definition, constant names are case-insensitive", true);
echo MESSAGE2."
"; //Output constant MESSAGE2
echo Message2."
"; //Output constant

$constant_name = "message2";
echo constant ($constant_name)."
"; //Output constant MESSAGE2
echo defined("MESSAGE")."
"; //If the definition returns true, the echo output displays 1
?>

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