🔴 Advanced · Lesson 20

include vs require in PHP

Q: When should I use require instead of include?

Use require for files the page cannot work without — like a database config or core functions. Use include for optional parts, like a sidebar, where a missing file should not crash the whole page.

In this lesson: Difference between include, require, include_once and require_once in PHP with error behaviour and examples.

Reusing code across files

Big projects split code into many files — a header, a footer, a config, shared functions. include and require pull one file into another so you write shared code once. They do almost the same thing; the difference is what happens when the file is missing.

💡 One-line rule: include = warning if missing (keeps going); require = fatal error (stops).

include

<?php
  include "header.php";   // pulls in header.php here
  echo "Page content";
?>

If header.php exists, its code runs at that point. If it is missing, PHP shows a warning but continues to "Page content". Good for optional pieces.

require

<?php
  require "config.php";   // essential file
  echo "This only runs if config.php loaded";
?>

If config.php is missing, require throws a fatal error and the script stops immediately — the echo never runs. Use it for files the page truly needs.

The key difference

Point	include	require
If file missing	Warning	Fatal error
Script continues?	Yes	No, it stops
Best for	Optional parts	Essential files

Choose based on importance: a missing sidebar should not break the page (include), but a missing database config should stop everything (require).

include_once and require_once

These load a file only once, even if the line runs again.

<?php
  require_once "functions.php";
  require_once "functions.php";  // ignored the second time
?>

If a file defines functions or classes, including it twice causes a "cannot redeclare" error. The _once versions prevent this by loading it a single time.

Common mistakes

Using include for essential files. A missing config then only warns, and the page breaks later in a confusing way — use require.
Double-including function files. Causes "cannot redeclare"; use require_once.
Wrong file path. Paths are relative to the current script — use __DIR__ for reliability.
Expecting a return value. These load a file; to get a value, have the file return it and capture it.

Summary

include and require load one PHP file into another.
On a missing file, include warns and continues; require stops with a fatal error.
Use require for essential files, include for optional parts.
require_once/include_once load a file only once.
Use __DIR__ for reliable file paths.

Next, organise larger apps with autoloading, or structure code with the MVC pattern.

Frequently Asked Questions

What is the difference between include and require?

Both load another PHP file into the current one. The difference is on failure: if the file is missing, include only gives a warning and the script keeps running, while require gives a fatal error and stops the script. Use require for essential files.

When should I use require instead of include?

Use require for files the page cannot work without — like a database config or core functions. Use include for optional parts, like a sidebar, where a missing file should not crash the whole page.

What do include_once and require_once do?

They work like include and require but load the file only once, even if called again. This prevents "cannot redeclare" errors when the same file (with functions or classes) might be included more than once.

Why do I get a "cannot redeclare function" error?

It usually means a file with a function or class was included twice. Switch to require_once (or include_once) so PHP loads it only the first time.

Can I include a file inside a function?

Yes. When you include a file inside a function, its variables follow the function's scope. This is one reason to prefer returning values or using clear structure over relying on included variables.

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