Introduction

In layman's terms, when something is said to be constant, it means that it cannot be altered in any way, even over time. It is not any different from constants in the PHP context. In fact, constants here are names or identifiers that when defined, cannot be altered. Shortly, we will dive more into its intricacies.

What we will cover

1. What a PHP Constant is.
2. PHP Constant naming conventions
3. How to define a PHP Constant.
4. Differences between Constants and Variables.

Prerequisites

Before getting into it, you at least have to know the following concepts and install the following tools in your machine.

1. Possess a basic PHP language understanding.
2. Installed xampp which you could get from here or wampp from here. The two software run the PHP scripts.
3. Have installed a text editor you prefer. I will be using VS Code to run basic code from this article. You could install it from this link.

Ready to learn? Let's begin!

What a PHP Constant is

From the above-covered introduction, constants can be compared to but not the same as variables. Unlike variables, once we initialize or define a constant, we cannot change the value attached to it when executing a script at any point in time. Of course, there exists an exception for this condition when we use (magic constants) instead. We will not cover magic constants in this article but you could find content on the same from here that explains magic constants concept profoundly. Due to their immutable nature, PHP Constants come in handy when defining data like configuration settings including the website's base URL, database username, and passwords, to mention but a few.

PHP Constants naming conventions

It is important to note that PHP constants are case-sensitive. Also, when naming constants, they must begin with either a letter or an underscore(_). Next, when we get to create a constant, we will also see that the constant names are always in uppercase.

Defining the PHP Constant

Now that we have some basic understanding of what a constant is, let's see how we could create one. The define() function is what we use when creating it. Three parameters are passed to it and these are:

1. Name: This references to what you call your defined constant.
2. Value: The value that is attached to the constant name.
3. case_insensitive: This is optional as a passed parameter and implies that a constant is case-insensitive. This parameter has two values: True and False. The default value for this parameter is false implying the constant being case-sensitive and true if case-insensitive.

   Remember constant names are always written in uppercase when defining and accessing their values. Therefore, when this parameter is set to true, you could write it in lowercase when accessing its value. Soon we will see this statement making better sense.

Below is the syntax used:

define(name, value, case_insensitive)

<?php
  //Defining a case-sensitive constant
  define("NAME", "Neema Muganga");
  echo NAME, "\n"; //Notice here, we write the constant name in uppercase as is required. By default, it is case-sensitive.

  //Defining a case-insensitve constant
  define("NAME", "Neema Muganga", true);

  //because we passed true for the case-insensitive parameter, we could echo the value of the constant with the name written in lowercase.
  echo name;
?>

The above code will output:

Neema Muganga
Neema Muganga

In this context, I will introduce the constant() function that is used to retrieve the value of the constant by passing in the name of the constant as a parameter. This function is especially useful when we need to access a constant value whose name is not known.

Therefore, in a different example code, the syntax will be as below:

<?php
  //Defining the Constant
  define("CITY", "Mombasa");

  echo CITY, "\n";

  //this will output same value as echo statement above
  echo constant("CITY");
?>

The output will be:

Mombasa
Mombasa

So far so good. Now we know what a constant is, why and when it could be used, how to create one, and how to finally access its value. Should the concept still feel unclear, please go through the article a second time and everything will make perfect sense.

Lastly, we will see how different constants are from variables just to clear any possible uncertainties when it comes to these two concepts.

Differences between Constants and Variables

1. First, we have already seen that, unlike variables that start with the '$' sign, constants begin with either an underscore '_' or a letter.
2. We also mentioned that constant values remain unchanged once defined, unlike variables whose value can be changed to something different.
3. Defining a constant is only done using the define() function, while we could define variables using assignment statements of values attached to the variable names that begin with a '$' sign.
4. In terms of scope, constants maintain a global scope. That is, they can be accessed from anywhere within a script. Variables on the other hand have their scope depending on where they have been defined in the script. Either globally or locally. You could read more about scope in PHP from this link to understand this distinction better.

That was fun! Now I hope you got the PHP constants concept well. Remember, you learn best by doing.

Happy Coding!