In PHP, what is the main difference between include and require when you bring in an external file?

Introduction / Context:
PHP provides multiple statements for including external files, notably include and require. At first glance they seem similar because both insert the contents of another file into the current script. However, their behaviour when a file is missing or cannot be loaded is different, and this difference is important for error handling and script stability.

Given Data / Assumptions:

• The script uses either include or require to import code from another file.
• The external file path may or may not be valid at runtime.
• We care about how PHP reacts to an inclusion failure.

Concept / Approach:
Both include and require evaluate the specified file and bring its definitions into the current script. The key distinction is in error severity when the file cannot be found. require treats a missing file as a critical error and triggers a fatal error, which stops script execution. include treats the problem as a warning and allows the script to continue running, which may or may not be desirable depending on the situation. This difference helps you choose between include for optional files and require for mandatory dependencies.

Step-by-Step Solution:
Step 1: Recall from documentation that require is stricter in handling errors than include.Step 2: If require fails to locate the file, PHP issues a fatal error and terminates execution, preventing further code from running.Step 3: If include fails, PHP issues a warning but continues executing the remaining script, which might lead to undefined functions or variables later.Step 4: Since the question asks for the main difference, focus on this error handling behaviour.Step 5: Option A captures this distinction accurately and is therefore the correct choice.

Verification / Alternative check:
You can test this by writing two scripts that attempt to load a non existent file. One script uses require "missing.php"; and the other uses include "missing.php";. Running the first script will show a fatal error and stop execution, while the second will show a warning and then continue to any code that follows, clearly demonstrating the difference.

Why Other Options Are Wrong:
Option B incorrectly claims that include is limited to PHP files and that require can load arbitrary types; both can load any file, though content type matters for interpretation. Option C inverts the concepts of runtime and compile time in a way that does not reflect PHP behaviour. Option D wrongly states that there is no difference, which contradicts both documentation and observed behaviour.

Common Pitfalls:
Developers sometimes use include in places where missing files should be treated as fatal, which can lead to scripts running in an inconsistent state. Others overuse require in optional feature areas where a missing file should not break the entire application. Good practice is to use require for essential configuration or library files and include for optional templates or components.

Final Answer:
require generates a fatal error and stops script execution if the file cannot be loaded, while include generates a warning and allows the script to continue running.