Deeply understand the order of require/include in php

 ini_set("include_path", ".:path_to_subdir"); // means including two paths, one '.' means the current path, and path_to_subdir means Absolute or relative path of subdir
 require("2.php");
?>

And in 2.php:

 require("3.php");
?>

and 3.php in the root directory prints "root", in the subdir directory The 3.php below prints out "subdir";

Now, here comes the question: 1. When running 1.php in the root directory, what output will you get? 2. Run 1.php in the upper directory under subdir. What output will you get? 3. When canceling the current directory path in include_path (that is, include_path="path_to_subdir"), what will be the output of the above two questions? include_path in PHP

When PHP encounters the require(_once)/include(_once) instruction, it will first make the following judgment: Is the file path to be included an absolute path? If so, include it directly and end it. If not, enter another logic (after multiple calls, enter _php_stream_fopen_with_path after macro expansion) to find this file.

Next, in _php_stream_fopen_with_path, the following judgment will be made: Is the file path to be included a relative path (in the form of ./file, ../dir/file, replaced by "directory relative path" below)? If so, skip the logic of include_path and directly parse the relative path (introduced separately later).

A candidate directory list will be formed based on include_path and the path of the currently executed file. For example, for the example in the previous article, a candidate list will be formed as follows ".:path_to_subdir:current_script_dir"

Then, starting from the head of the candidate list, take out a path in the candidate list according to DEFAULT_DIR_SEPARATOR (the environment of this article is ":"), and then append the file name to be included to the end of this path and try. If successful Contains, then return, otherwise continue with the next candidate path.

Up to now, we can answer the 3 questions I raised at the beginning.

1. Because it is executed in the root directory, when 1.php includes 2.php, the second candidate path of include_path works (path_to_subdir), and path_to_subdir/2.php is found, while in 2. When php contains 3.php, the current working directory is under root, so when 3.php is included, the matching file is found in the first candidate path of include_path "." (current working directory), so we get The output is "root".

2. Same as 1, except that the current path is subdir, so the output is "subdir".

3. Because there is no include_path in the current path, when 2.php is included in 3.php when running in the root directory, path_to_subdir plays a role, so whether in root or subdir, you will get the output of "subdir" . And if you clear include_path in 2.php,

 ini_set("include_path", '');
 require("3.php");
?>

Then the current_script_dir will take effect , and at this time current_script_dir is the path of 2.php, so you will still get the output of "subdir".

Directory relative path

When using a directory relative path, the base point of the relative path is always the current working directory.

In order to illustrate the situation under directory relative paths, let’s look at another example. It’s still the above directory structure, but 1.php becomes:

 ini_set("include_path", "/");
 require("./subdir/2.php");
?>

2. php becomes:

 require("./3.php");
?>

If executed in the root directory, searching for 3.php in 2.php will be searched in the relative path of the current directory, so the output obtained is "root", and if executed under subdir, 1 of the upper-level directory will be searched. php(php -f ../1.php), will exit abnormally because "./subdir/2.php" cannot be found under subdir.

Summary

1. Because when using include_path and relative paths, performance will be related to the number of searches. In the worst case, if you have 10 include_paths, you may have to retry up to 11 times to find the file to be included, so , it is best to use absolute paths when absolute paths can be used.

2. Because the basedir of the directory relative path is always the current working path, if it is to be used, it needs to be related to the actual deployment path, so it is rarely used in practice (of course, there are also modules that use chdir to complete it).

3. In modular system design, the absolute path should generally be used within the module by obtaining the deployment path of the module (dirname(__FILE__), php5.3 and later provides the __DIR__ constant).