PHP INTERVIEW · Episode 03

What’s the difference between include and require?

And when to use include_once / require_once

ep3_require_vs_include.php

<?php
// Your code will go here.
include("includes/include-test.php"); //no error success
require("includes/include-test.php"); //no error success

include("includes/include-tests.php"); //warning, no success
require("includes/include-tests.php"); //fatal error, no success

Output

Output for this tutorial goes here.

Show the actual printed result of the code above.

The Question

Both include and require pull one PHP file into another so you can reuse code — headers, config, functions. The difference is what happens when the file is missing, and that’s exactly what interviewers want you to know.

Explanation

require is strict. If the file can’t be found, PHP throws a fatal error and stops the script right there — nothing after it runs.

include is forgiving. If the file is missing, PHP only emits a warning and the script keeps running.

So the rule of thumb: use require for files your app cannot run without (config, core classes, database connection), and include for optional pieces (a sidebar, a banner) where a missing file shouldn’t break the whole page.

The _once versions

require_once and include_once behave the same, but PHP will only load the file one time. Even if you call them again later, the file is skipped — which prevents “cannot redeclare function/class” errors. Use these for files that define functions or classes.

Key takeaway: require fails hard (fatal error, stops), include fails soft (warning, continues). The _once variants load the file at most once.

Common mistake: using include for a critical file — the script limps on in a broken state and throws confusing errors later, instead of failing fast where the real problem is.

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