PHP Constants are like variables, but they cannot be changed once they are defined. They are used to store values that do not change throughout the execution of a script. Constants are case-sensitive by default and are defined using the define()
function.
Constants are useful when you need to store a value that is used repeatedly throughout your code, such as a database username or password. By using a constant, you can ensure that the value is always the same and cannot be accidentally changed.
Here is an example of defining a constant:
<?php
define("DB_USERNAME", "myusername");
define("DB_PASSWORD", "mypassword");
?>
In this example, we have defined two constants: DB_USERNAME
and DB_PASSWORD
. These constants can now be used throughout our code without the risk of accidentally changing their values.
Constants can also be defined with arrays:
<?php
define("FRUITS", [
"apple",
"banana",
"orange"
]);
?>
In this example, we have defined a constant called FRUITS
that contains an array of fruit names. This constant can now be used throughout our code to reference the array of fruit names.
Constants can also be defined with expressions:
<?php
define("TODAY", date("F j, Y"));
?>
In this example, we have defined a constant called TODAY
that contains the current date in the format of "Month Day, Year". This constant can now be used throughout our code to reference the current date.
Constants can be used in the same way as variables:
<?php
echo "My database username is " . DB_USERNAME . ".";
echo "Today is " . TODAY . ".";
?>
In this example, we have used the echo
statement to output the values of the DB_USERNAME
and TODAY
constants.
Constants can also be used in function definitions:
<?php
function myFunction($param1, $param2 = DB_PASSWORD) {
// function code here
}
?>
In this example, we have defined a function called myFunction
that takes two parameters. The second parameter has a default value of the DB_PASSWORD
constant. This means that if the second parameter is not provided when the function is called, it will default to the value of the DB_PASSWORD
constant.
Constants can also be used in class definitions:
<?php
class MyClass {
const MY_CONSTANT = "Hello, world!";
}
?>
In this example, we have defined a class called MyClass
that contains a constant called MY_CONSTANT
. This constant can be accessed from within the class using the self::
keyword:
<?php
echo MyClass::MY_CONSTANT;
?>
Constants can also be used in switch statements:
<?php
switch ($variable) {
case DB_USERNAME:
// code here
break;
case DB_PASSWORD:
// code here
break;
default:
// code here
}
?>
In this example, we have used the switch
statement to compare the value of the $variable
variable to the values of the DB_USERNAME
and DB_PASSWORD
constants.
Constants are a powerful tool in PHP that can help you to write more efficient and maintainable code. By using constants, you can ensure that values that do not change throughout the execution of your script are always the same and cannot be accidentally changed.