Great question! Let’s unpack why both include paths work in your setup, and which approach is the more reliable,规范 choice.

First, let’s recap your directory structure for clarity:

project/
├─ index.php
└─ lib/
   ├─ file1.php
   └─ file2.php

Why both paths successfully find file2.php #

PHP has specific rules for resolving file paths when using include or require, and your two paths trigger different parts of those rules:

1. When you use include 'lib/file2.php' in file1.php
   This relative path is resolved against the current working directory—which is the directory of the entry script (your index.php in project/). So PHP looks for project/lib/file2.php, which exists, hence it works.

2. When you use include 'file2.php' in file1.php
   When you use just a filename without a path prefix, PHP follows this search order:

   • First, it checks directories listed in PHP’s include_path config.
   • If not found there, it checks the directory of the script making the include call (here, file1.php lives in project/lib/, so it finds project/lib/file2.php immediately).
   • If still no luck, it falls back to the current working directory.

That’s why both paths end up locating the correct file.

Which写法 is more规范? #

The most robust and future-proof approach is to use absolute paths relative to the current script, but let’s break down the options:

• Avoid relying on the current working directory: Using include 'lib/file2.php' depends entirely on index.php being the entry point and the working directory staying fixed at project/. If you ever run the script from a different directory (via CLI) or use chdir() elsewhere, this path will break.

• Using just include 'file2.php' is better but still risky: While it works here, this relies on PHP’s default search order. If someone adds a file2.php to a directory in include_path, your code might accidentally include the wrong file instead of the one in lib/.

• The gold standard: Use __DIR__ for absolute path clarity
  For file1.php, replace your include line with:

  include __DIR__ . '/file2.php';
  

  __DIR__ is a magic constant that returns the absolute directory path of the script it’s written in (so for file1.php, that’s something like /your/absolute/path/project/lib/). Combining it with /file2.php creates an unambiguous absolute path to the file. This works no matter what the current working directory is, or how include_path is configured.

  If you’re stuck on PHP 5.2 or older (which you shouldn’t be, but just in case), use dirname(__FILE__) instead of __DIR__.

Final takeaway #

Both paths work thanks to PHP’s file resolution logic, but they’re not equally reliable. For maintainable, bug-free code, always use absolute paths built with __DIR__ to eliminate any ambiguity about where PHP should look for files.

内容的提问来源于stack exchange，提问作者user3425506