What is the difference between include, require, include_once, and require_once in PHP?

Here’s the explanation of include, require, include_once, and require_once in PHP:

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1. include

• Includes and evaluates the specified file.
• If the file is not found, it throws a warning but continues the execution of the script.

Example:

include 'file.php';
echo "This will still run even if file.php is missing.";

Use Case:

• Use include when the file is optional, and your application can still run without it.

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2. require

• Includes and evaluates the specified file.
• If the file is not found, it throws a fatal error and stops script execution.

Example:

require 'file.php';
echo "This will NOT run if file.php is missing.";

Use Case:

• Use require when the file is essential, and the application cannot run without it.

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3. include_once

• Same as include, but it ensures that the file is included only once during the script execution.
• If the file is already included, it will not include it again.

Example:

include_once 'file.php';
include_once 'file.php'; // This will not include file.php again.

Use Case:

• Use include_once when the file is optional, but you want to avoid including it multiple times (e.g., when defining functions).

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4. require_once

• Same as require, but it ensures that the file is included only once during the script execution.
• If the file is already included, it will not include it again.

Example:

require_once 'file.php';
require_once 'file.php'; // This will not include file.php again.

Use Case:

• Use require_once when the file is essential, and you want to ensure it’s included only once (e.g., configuration files or class definitions).

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Comparison Table

Directive	Behavior on Missing File	Multiple Inclusions Allowed?	Use Case
include	Warning (Script continues)	Yes	Optional files
require	Fatal error (Script stops)	Yes	Essential files
include_once	Warning (Script continues)	No	Optional files (Avoid duplicates)
require_once	Fatal error (Script stops)	No	Essential files (Avoid duplicates)

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Best Practices:

• Use require or require_once for files your application cannot function without (e.g., configuration, database connection).
• Use include or include_once for non-critical files (e.g., optional templates or headers).

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