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If you want to use the function defined in the file in a PHP script, you need to use one of the functions include(), include_once(), require() and require_once() to load the function library file in the script. In the previous chapter, we talked about How to use custom function libraries in PHP. This chapter will specifically talk about the differences and connections between require() and include().
PHP provides two methods for including external files: require() and include() are common methods in server-side references (Server Side Includes).
require is used like require("MyRequireFile.php"); . This function is usually placed at the front of the PHP program. Before the PHP program is executed, it will first read in the file specified by require and make it a part of the PHP program web page. Commonly used functions can also be introduced into web pages in this way. When require includes a file, it is included absolutely. If the included file does not exist, a fatal error will be generated. After the error occurs, the following script program will not be executed. In addition, because the require statement is equivalent to adding another The content of a source file is completely copied to this file, so it is generally placed at the beginning of the source file to reference the public function files and public class files that need to be used.
include Usage methods such as include("MyIncludeFile.php");. This function is generally placed in the processing part of flow control. The PHP program webpage only reads the include file when it reads it. In this way, the process of program execution can be simplified. The included files in include are relative. If the included file does not exist, a warning warning will be generated, but the following script program will continue to execute. In addition, files included in include are generally written during program execution.
The difference between include() and require()
The difference between include() and require() This is believed to be the most basic problem in PHP. These two The uses are exactly the same, it doesn’t necessarily have to be which one is placed at the front and which one is placed in the middle. The most fundamental difference between them is the way they handle errors.
require(): If the file does not exist, a fatal error will be reported. The script will stop executing.
include(): If the file does not exist, a warning will be given, but the script will continue. The performance of
php's require() is similar to include(). The difference is that for include(), the file must be read and evaluated every time when include() is executed; while for require(), the file is only processed once (in fact, the file content replaces require () statement). This means that if you have code that contains one of these instructions and code that may be executed multiple times, it is more efficient to use require(). On the other hand, if you read a different file each time the code is executed, or have a loop that iterates through a set of files, use include() because you can set a variable for the file name you want to include. When the argument is Use this variable when including().
Some other connections and differences to add
1. include has a return value, but require does not.
2. include() includes and runs the specified file. When the processing fails, include() generates a warning. The imported program code will be executed, and these programs will have and call the source file when executed. to the same variable scope as the include() statement. You can import static pages from the same server.
3. The function of include_once() is almost the same as include()
The only difference is that include_once() will first check whether the file to be imported is already in other files in the program The place has been imported, if so, it will not be imported again (this function is sometimes very important, for example, if the function to be imported declares some functions that you have defined yourself, then if it is repeated in the same program When importing this file, an error message will occur the second time it is imported, because PHP does not allow functions with the same name to be declared twice).
4. require() will read the contents of the target file and replace itself with the read contents. If the processing fails, require() will cause a fatal error.
5. Like include_once(), require_once() will first check whether the content of the target file has been imported before. If so, the same content will not be imported again.
6. require is an unconditional inclusion, that is, if require is added to a process, require will be executed first regardless of whether the condition is true or not.
The following example fully illustrates the difference between these two functions:
Write two php files named test-include.php and test-require .php, please note that there should not be a file named test1.php in the same directory.
test-include.php
Run the above code, because the test1.php file is not found, we see the error message. At the same time, abc is displayed below the error message. What you see may be similar to the following:
Warning: include(test1.php) [function.include]: failed to open stream: No such file or directory in D:\www\test-include.php on line 3 Warning: include() [function.include]: Failed opening 'test1.php' for inclusion (include_path='.;C:\php5\pear') in D:\www\test-include.php on line 3 abc
test-require.php
运行上面的代码，因为没有找到test1.php文件，我们看到了报错信息，但是，报错信息的下边没有显示abc，你看到的可能是类似下边的情况：
Warning: require(test1.php) [function.require]: failed to open stream: No such file or directory in D:\www\test-require.php on line 3 Fatal error: require() [function.require]: Failed opening required 'test1' (include_path='.;C:\php5\pear') in D:\www\test-require.php on line 3
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