Proper usage of constants in PHP - php

Question

So I have recently learned about constants in PHP. fortunately there is not really anything to learn on them, they appear to be just an entity which stores value of something, just like a variable.
The only different that I can see (Correct me if I am wrong) is that a constant cannot be assigned a new value (that is it?). Which makes sense asthetically for example if you are making a config file with configurations that should not be changed.
But surely it brings other benefits/usage than just using a variable anyway?
In what situations should I be choosing to use constants?

Answer 1

If you find yourself setting a variable for convenience and never changing it during a script, the chances are you should be using a constant instead. Constants are like variables except that once they are defined they cannot be undefined or changed - they are constant as the name suggests. In many languages, constants are faster than variables and so are recommended, but this is not the case as much in PHP - although they are perhaps a small amount faster, the primary advantage to using constants is the fact that they do not have a dollar sign at the front, and so are visibly different from variables. Furthermore, constants are automatically global across your entire script, unlike variables.
To set a constant, use the define() function - it takes two parameters, with the first being the name of the constant to set, and the second being the value to which you wish to set it. For example, this following line of code sets the variable CURRENT_TIME to be the return value of the time() function, then prints it out:
define("CURRENT_TIME", time());
print CURRENT_TIME;
Note that it is not $CURRENT_TIME or Current_Time - constants, like variables, are case sensitive, but unlike variables they do not start with a dollar sign. You can change this behaviour by passing true as a third parameter to define(), which makes the constant case-insensitive:
define("CURRENT_TIME", time(), true);
print Current_TiMe;
There are two helpful functions available for working with constants, and these are defined() and constant(). The defined() function is basically the constant equivalent of isset(), as it returns true if the constant string you pass to it has been defined. For example:
define("CURRENT_TIME", time(), true);
if (defined("CURRENT_time")) {
/// etc }
Note that you should pass the constant name into defined() inside quotes.

Answer 2

A key characteristic of constants is that they behave like functions and classes, in the sense that they need to be defined when you try to use them, otherwise PHP will produce an error; and that they're global and unconcerned by scope. This makes them ideal for flags in function parameters, where the value is relatively unimportant, yet requires unique identification. Just look at how PHP itself uses constants:
json_encode($foo, JSON_UNESCAPED_UNICODE)
pathinfo($bar, PATHINFO_FILENAME)
$pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION)
What would the alternatives be?
$pdo->setAttribute('errmode', 'exception')
Strings cannot be checked for existence the same way constants can, typos are easier and require more manual error handling inside the function, and strings cannot be introspected the same way you can introspect PDO for its constants, and hence discoverability of available options is reduced.
$pdo->setAttribute(1, 2)
Magic numbers are obviously even worse, with even less meaning and the same lack of enforcement and discoverability.
In these cases, constants are ideal. They provide an enforced, readable identifier for a meaning; the actual value the constant holds is relatively unimportant.

Answer 3

Constants are for when someone else might read and try to install your code, but doesn't know what bits they should change to add their own details in. It's especially useful if you don't have an installation script, as the new admin will need to set their own details.
Example:
You need to install a new script on your website that uses a database of some kind. You'd open up the main file, and edit the constants that you need to add in your database connection details. Because they're constants, you know they shouldn't change - And they shouldn't, otherwise your database wouldn't work! If you use variables, the values could be accidentally changed in the script. It's also clearer to know which parts you have to change - and as they're usually at the start of a document or in a separate file easy to identify as well.