php中的include，require，include_once，require_once

In PHP, you can use include, require, include_once, require_once statements to reference files. The following article briefly introduces these statements to you. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

Use the include statement to reference the file

Note: When using include to apply external files, only when the code is executed to the include statement, the external The file will be introduced and read. When an error occurs in the referenced external file, the system will only give a warning, and the entire php file will continue to execute downwards.

<?
include("top.php");
include("main.php");
include("bottom.php");
?>

Use the require statement to reference the file

Before the php file is executed, the php parser will replace the require statement with the entire content of the referenced file, and then match it with require Other statements besides the statement form a new php file, and finally press the new php file to execute the program code.

Note: Because the require statement is equivalent to completely copying the contents of another source file into the file, it is generally placed at the beginning of the source file to reference the public function files and public files that need to be used. class files, etc.

The difference between include statement and require statement

When using require statement to call a file, if the called file is not found, the require statement will output an error message and terminate immediately Script processing. The include statement will output a warning when the file is not found and will not terminate script processing.

When using the require statement to call a file, as soon as the program is executed, the external file will be called immediately; when calling the external file through the include statement, the external file will only be called when the program executes the statement.

Apply the include_once statement to reference the file

Apply the include_once statement to check whether the file has been applied in other parts of the page before importing the file. If so, then The file will not be referenced repeatedly and can only be referenced once by the program.

For example: There are some custom functions in the file to be imported. If you import this file repeatedly in the same program, an error will occur during the second import because PHP does not allow functions with the same name. Declared repeatedly

The require_once statement is used to reference the file

require_once statement is an extension of the require statement. Its function is basically similar to the require statement. The difference is that when applying require_once statement, it will first check whether the file to be referenced has been referenced elsewhere in the program. If so, the file will not be called repeatedly.

For example: if two identical files are referenced in the same page by applying the require_once statement at the same time, only the first file will be executed during output, and the second referenced file will not be executed.

The difference between include_once and require_once statements

The include_once statement generates a warning when an error occurs when calling an external file during script execution, while the require_once statement causes a fatal error .

The purpose is to ensure that an included file can only be included once. Using these two statements can prevent the same function library from being accidentally included multiple times, resulting in repeated definitions of functions and generating errors.

This article is reproduced from: https://blog.csdn.net/qq_28602957/article/details/50945797

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