Several methods of file reference in php

Use include statements to reference files

Note: When using include to apply external files, the external files will be introduced and added only when the code is executed to the include statement. Reading, when an error occurs in the referenced external file, the system only gives a warning, and the entire php file continues 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 it with the entire content of the referenced file require statement, and then form a new php file with other statements other than the require statement, 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 you call The file is not found, the require statement will output an error message and terminate script processing immediately. 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.

Use include_once statement to reference the file

Use include_once statement to check the file before importing it Whether it has been applied in other parts of the page, if so, the file will not be referenced repeatedly, and the program can only reference it once.

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. It is repeatedly declared

When the require_once statement is used to reference the file

require_once statement is an extension of the require statement, it The function is basically similar to the require statement. The difference is that when applying the 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 the use of include_once and require_once statements

The include_once statement occurs when calling external files during script execution On error, a warning is generated, and the require_once statement results in 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.

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