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Notes the entire program and change the value at each instance. You only need to change it at the
beginning of the program where you set the initial value for the constant.
Have a look at the example where we use the define function to set the initial value of a constant:

php

// first we define a constant PASSWORD

define(“PASSWORD”,”admin”);
echo (PASSWORD); // will display value of PASSWORD constant, i.e. admin

echo constant(“PASSWORD”); // will also display admin
echo “PASSWORD”; // will display PASSWORD

?>
PHP also provides a number of built-in constants for you. “__FILE__”, for example, returns the
name of the file currently being read by the interpreter. “__LINE__” returns the line number of
the file. These constants are useful for generating error messages. You can also find out which
version of PHP is interpreting the script using the “PHP_VERSION” constant.

In PHP, you can define names, called constants, for simple values. As the name implies, you
cannot change these constants once they represent a certain value. The names for constants
have the same rules as PHP variables except that they don’t have the leading dollar sign. It is
common practice in many programming languages—including PHP—to use uppercase letters
for constant names, although you don’t have to. If you wish, which we do not recommend, you
may define your constants as case-insensitive, thus not requiring code to use the correct casing
when referring to your constants.

Only use case-sensitive constants both to be consistent with accepted coding
standards and because it is unclear if case-insensitive constants will continued
to be supported in future versions of PHP. Unlike variables, constants, once
defined, are globally accessible. You don’t have to (and can’t) redeclare them in
each new function and PHP file. To define a constant, use the following function:

Define (“CONSTANT_NAME”, value [, case_sensitivity])

Where:
“CONSTANT_NAME” is a string.

value is any valid PHP expression excluding arrays and objects.

case_sensitivity is a Boolean (true/false) and is optional. The default is true.
An example for a built-in constant is the Boolean value true, which is registered as case-insensitive.

Here’s a simple example for defining and using a constant:

define(“MY_OK”, 0);
define(“MY_ERROR”, 1);

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