Include Functions in PHP

Include functions are used to insert the content of one PHP file into another PHP file. They help in code reusability, easy maintenance, and modular programming.

Why Include Functions are Needed?

Without include:

• Same header/footer code repeated
• Difficult maintenance

With include:

• One common file
• Changes reflect everywhere

Exam Definition: Include functions allow reuse of PHP code by inserting external files into the current script.

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1. include() Function

The include() function includes a file and continues execution even if the file is not found.

Syntax

include("filename.php");

Example

include("header.php");
echo "Main Content";

If File is Missing

• Warning is shown
• Script continues execution

Exam Point: include() generates a warning but does not stop script execution.

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2. include_once() Function

• Includes a file only once
• Prevents duplicate inclusion

Syntax

include_once("filename.php");

Example

include_once("config.php");
include_once("config.php");   // Will not be included again

Use Case: Database connection files.

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3. require() Function

The require() function includes a file and stops execution if the file is not found.

Syntax

require("filename.php");

Example

require("db.php");
echo "Connected";

If File is Missing

• Fatal error occurs
• Script stops execution

Exam Line: require() is used for critical files.

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4. require_once() Function

• Includes a file only once
• Stops execution if file is missing

Syntax

require_once("filename.php");

Example

require_once("config.php");

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Difference Between Include and Require

Difference Between *_once Versions

Exam Tip: Use require_once for database configuration files.