Constants

Php-mysql Course

Constants are similar to variables only in the sense that are used to store values, but otherwise, constants and variables differ in many ways.
To create a constant, it is used the define() function instead of the assignment operator (=):

define("CONSTANT_NAME", 'value', [case-insensitive]);

-The case-insensitive parameter at the end of the definition is optional (a boolean value: true or false), and by default is false, meaning that the defined constant is in fact case-sensitive.
- Constants do not use the initial dollar, their name must start with either a letter or an underscore character, followed by letters, numbers, or underscores.
- By default and convention, a defined constant is uppercase, although this is not required.
- A constant can only be assigned a scalar value (a string or a number). And unlike variables, a constant's value cannot be changed.
- To access the value of a constant, make reference to its name.

The following example creates two constants and attempts to recreate one of them.

<?php
define("BROTHERS", 'Victor, Alex');
define("COUSIN", "Vasile", true);                // defined case-insensitive

echo 'My brothers: '. BROTHERS. '<br />';        // My brothers: Victor, Alex

// Try to redefine the BROTHERS constant
define("BROTHERS", 'Relu');                      // Notice: Constant BROTHERS already defined in ...

echo 'My cousin: '. COUSIN. '<br />';            // My cousin: Vasile

// assign the constant's value (the one case-insensitive) to a variable
$cname = Cousin;
echo 'My cousin: '. $cname;                     // My cousin: Vasile
?>

You cannot put the constant within quotation marks:
                echo "My brothers: BROTHERS";     results:   My brothers: BROTHERS

The scope of a defined constant is global, meaning it can be used in any defined function or class that is also part of that code file, including any other included files or functions.

• If you want to check if a constant is defined, you can use the defined("CONSTANT") function. This function returns 1 if "CONSTANT" is defined, otherwise returns false.

<?php
define("BROTHERS", 'Victor, Alex');
echo defined("BROTHERS");           // 1
?>

- The parameter with the name of the constant must be added within double quotes.