In a PHP script index.php, can you include the same file xyz.php twice, and what is the effect?

Introduction / Context:
File inclusion is a core feature of PHP and developers often split applications into multiple reusable files. Understanding how include behaves when the same file is included more than once is important for avoiding subtle bugs, especially redeclaration errors. Interview questions about include, include_once, require, and require_once are common because they reveal whether a candidate understands how PHP loads code and how to structure reusable libraries safely.

Given Data / Assumptions:

• We have a main script index.php that uses include to bring in code from another file xyz.php.
• The include statement is used at two different points in the same script.
• xyz.php may contain function definitions, class definitions, constants, and executable statements.
• The question is about plain include, not include_once or require_once.

Concept / Approach:
The include statement in PHP behaves like a simple textual inclusion. When PHP encounters include with a valid path, it reads and executes the contents of that file at that point in the script. If you call include for the same file more than once, PHP will load and execute the file each time. This is allowed, but if the file contains definitions that must be unique, such as functions or classes, executing it multiple times will usually cause fatal errors about redeclaration. The *_once variants exist precisely to prevent duplicate inclusion.

Step-by-Step Solution:
Step 1: Consider index.php with two include statements that both reference xyz.php at different locations in the code. Step 2: When PHP executes the script and reaches the first include, it reads xyz.php and executes its contents, which may output HTML or define reusable functions. Step 3: Later, when PHP reaches the second include, it again reads and executes xyz.php, because include has no built in memory of previous inclusions. Step 4: If xyz.php contains only executable code such as echo statements, they will run twice, possibly producing duplicate output. Step 5: If xyz.php defines functions or classes, the second inclusion will usually raise a fatal error because PHP does not allow the same function or class name to be defined twice in the same request. Step 6: To avoid multiple inclusion problems, you can use include_once, require_once, or implement manual checks, but plain include itself will execute files each time it is called.

Verification / Alternative check:
A simple test demonstrates the behaviour. Create xyz.php with a line that echoes a message and define a function. In index.php, include xyz.php twice. When you run index.php, you will see the echoed message twice and a fatal error about the function already being declared. If you change include to include_once, the file is executed only once and the error disappears. This confirms that plain include processes the file at each call without preventing duplication.

Why Other Options Are Wrong:
Option b is wrong because PHP does not raise a syntax error just for including the same name twice; errors occur only when duplicate definitions conflict at runtime. Option c is incorrect because automatic prevention of duplicate inclusion is provided by include_once and require_once, not by normal include. Option d is unrealistic because PHP does not require the file to be empty; the effect depends on the content of the included file, not on a rule that only empty files can be included twice.

Common Pitfalls:
A common pitfall is placing configuration or function library includes inside loops or conditional blocks, which may cause multiple inclusion inadvertently and lead to redeclaration errors. Another issue is forgetting to use include_once or require_once for framework bootstrap files that must only be loaded once per request. Good practice is to decide which files should be included exactly once and use the *_once statements for them, while using include or require for view templates or other code that is safe to execute multiple times.

Final Answer:
Yes, you can include the same file twice with include and the code in xyz.php will run each time, but doing so can cause redeclaration or duplicate output problems, which is why include_once or require_once are often preferred for library files.