PHP provides include, require, include_once, and require_once to insert external files; 2. include issues a warning and continues if the file is missing, suitable for non-critical content; 3. require causes a fatal error and stops execution if the file is not found, ideal for essential files like configurations; 4. include_once and require_once prevent multiple inclusions, avoiding redeclaration errors; 5. Use require for critical files and include for optional ones, adding _once when duplicates must be avoided.
PHP lets you include or require external files in your script using four main constructs: include, require, include_once, and require_once. These are useful for reusing code like headers, footers, functions, or configuration files.
include
Use include when you want to insert the content of one file into another. If the file is not found, PHP will throw a warning but continue executing the rest of the script.
include 'file.php';- If file.php is missing, the script shows a warning and keeps running.
- Suitable for non-critical files (e.g., sidebars, banners).
require
Use require for essential files. If the file is missing, PHP will trigger a fatal error and stop execution.
require 'config.php';- If config.php is not found, the script halts completely.
- Ideal for critical files like database connections or authentication logic.
include_once and require_once
These ensure a file is included only once, even if the statement appears multiple times. Prevents function redefinitions or duplicate class declarations.
-
include_once 'functions.php';– includes file once, ignores repeated calls. -
require_once 'Database.php';– same as require, but prevents duplicates. - Useful when working with libraries or utility files that define functions or classes.
Choose the right method based on whether the file is critical and whether it might be included more than once. Basically, use require for essential files and include for optional ones. Add _once when duplication could cause errors.
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