DIFFERENCES IN A NUTSHELL

• require Where a file is required by your script, will throw a fatal error if the specified file is not found (or is not readable). Stops any further processing on a missing file.
• include Where an optional file is needed by your script. Will throw a warning if the specified file is not found (or is not readable). Will continue processing on a missing file.
• require_once Same as require, but will checks if the specified file has already been loaded before.
• include_once Same as include, but will checks if the specified file has already been loaded before.

INCLUDE EXAMPLE

As you code ninjas already know, we use include to load another file into our script. So let us begin the example with a simple dummy class file.

Then, we use include to load it in our main script.

speak();
?>

Yep, include is that simple, but take extra note on how it continues to run even if the specified file is not found.

REQUIRE EXAMPLE

require is the strict sibling of include, and it will halt all further processing on a missing file.

speak();
?>

INCLUDE_ONCE AND REQUIRE_ONCE

speak();
?>

WHEN TO USE WHAT?

Now that you know the differences between all of these siblings, when should we use each one? Well, it all depends on the situation.

• Use require if the external file is critical to further processing. E.g. Loading a library file.
• Use include if the external file is optional for further processing. E.g. Loading a log file or some additional statistic.
• require_once and include_once if the file may have been loaded somewhere else before.
• There has been some past debate that require_once and include_once don’t perform very well. That they are slow, has to check if the file is already loaded – But the modern versions of PHP proved otherwise. While they do not perform as well, but they are still very decent. 
• I will personally not recommend using require_once and include_once though… It is kind of a bad lazy way to prevent good programming practices and structuring.

CAN LOAD ANY FILE TYPE

Notice that I use “include a file”, and not “include a script” above? Yep, we can pretty much load any kind of files with require and include – 

html, body {
  font-family: arial;
  font-size: 2em;
}

  This is formatted in HTML.

This is just plain text.

SEQUENCE DOES MATTER

Common newbie mistake – You have to remember that PHP is read sequentially, from top to bottom, left to right. So please don’t use a class before you load it.

speak();
require "1-class.php";
?>

speak();
?>

INCLUDE NOT WORKING – CASE SENSITIVE

So, you have properly included the file in a Windows setup and it works perfectly. But once on a Linux machine, it fails miserably. Just why is that so? Two words – case sensitive.

THE QUICK INFOGRAPHIC

REFERENCES

• Official PHP manual – require
• Official PHP manual – include
• require_once
• include_once

ABSOLUTE VS RELATIVE PATH

Still having trouble with “missing files”? That may be a file path issude, and check out why in my other guide:

Absolute and Relative Paths in PHP – A Comprehensive Guide

QUICK START

Skipped the entire tutorial? There is no database setup nor anything “special”, so just download and unzip into a temporary folder – Then run through each of the example files.

SOURCE CODE DOWNLOAD

Click here to download the source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.
 

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