Please stay away from include_once and require_once_PHP tutorials

Try to use include instead of include_once. The reason is that include_once needs to query the loaded file list to confirm whether it exists, and then load it again.

It is true that this reason is correct, but what I want to talk about today is another reason.

We know that to determine whether a file is loaded, PHP needs to get the open_path of the file, which means, for example:

set_include_path("/tmp/:/tmp2/");
include_once("2.php");
?>

When PHP sees include_once "2.php", it does not know what the actual path of this file is, and it cannot judge whether it has been loaded from the loaded file list, so in include_once In the implementation, it will first try to parse the real path of the file (for ordinary files, this parsing is just like checking getcwd and the file path, so if it is a relative path, it will generally not succeed). If the parsing is successful, it will look for EG (include_files) , if it exists, it means it has been included and returns, otherwise open the file to get the opened_path of this file. For example, in the above example, this file exists in "/tmp2/2.php".

Then, get After getting this opened_path, PHP goes to the loaded file list to find out whether it is included. If it is not included, then compile directly, and there is no need to open the file anymore.

1. Try to parse the absolute path of the file, If the parsing is successful, check EG (included_files) and return if it exists. If it does not exist, continue
2. Open the file and get the open path of the file (opened path)
3. Use the opened path to search for EG (included_files) , whether it exists, return if it exists, continue if it does not exist
4. Compile file (compile_file)

In most cases, this is not a problem, but the problem lies when you use APC...

When using APC, APC hijacks the compile_file pointer to get the compilation result directly from the cache, avoiding opening the actual file and avoiding the system call for open.

However, when you use include_once in your code, PHP has already tried to open the file before compile_file, and then enters the compile file hijacked by APC. This will generate an additional open operation. To solve this problem, APC introduced include_once_override. When include_once_override is turned on, APC will hijack PHP's ZEND_INCLUDE_OR_EVAL opcode handler, determine the absolute path of the file through stat, and then if it is found that it has not been loaded, it will rewrite the opcode to include. , make a tricky solution.

But, unfortunately, as I said, APC’s include_once_override implementation has not been good, and there will be some undefined problems, such as:

< ?php
set_include_path("/tmp");
function a($arg = array()) {
include_once("b.php");
}

a ();
a();
?>

Then, our b.php is placed in "/tmp/b.php" with the following content:

< ;?php
class B {}
?>

Then when apc.include_once_override is turned on, continuous access will get the following error:

Fatal error - include () : Cannot redeclare class

Excluding these technical factors, I have always believed that we should use include instead of include_once, because we can completely plan by ourselves, and a file will only be loaded once. You can also use the help of Automatic loading can do this.

If you use include_once, it can only prove that you have no confidence in your own code.

So, I suggest you not to use include_once