What will be the output of the following PHP code?

In PHP, a constant is a name for a value that cannot be changed during the execution of a script. Constants are similar to variables, but once defined, they cannot be changed or undefined.  Here are some rules for constants in PHP: Constants are case-sensitive and are usually written in uppercase. Constants defined using the define() function may be case-insensitive prior to PHP 8.0.0. Constants are automatically global and can be used across the entire script. Constants can be defined on a per-class basis. Class constants can be redefined by a child class. However, as of PHP 8.1.0,... Show more

In PHP, a constant is a name for a value that cannot be changed during the execution of a script. Constants are similar to variables, but once defined, they cannot be changed or undefined. 

Here are some rules for constants in PHP:
Constants are case-sensitive and are usually written in uppercase.
Constants defined using the define() function may be case-insensitive prior to PHP 8.0.0.
Constants are automatically global and can be used across the entire script.
Constants can be defined on a per-class basis.
Class constants can be redefined by a child class. However, as of PHP 8.1.0, class constants cannot be redefined by a child class if it is defined as final.

PHP has nine predefined constants that change value depending on where they are used. These constants are called "magic constants".
Constants can be defined in one of two ways: as a c-constant using a const-declaration, or as a d-constant by calling the library function define. 

To create a constant, you can use the define() function. The syntax for define() is:  define(name, value, case-insensitive) [2] 
 
To use a constant inside a class definition, you can use the PHP keyword const and initialize it with a scalar value. Scalar values include: Boolean, Int, Float, String, and Array (PHP 5.6+). 
You can use a concatenation operator ('.') to echo more than one variable or constant on a line. 

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