include() and require() : Statement includes and runs the specified file.
include() generates a warning and require() results in a fatal error. In other words, if you want to stop processing the page when you encounter a lost file, use require(). include() is not the case, the script will continue to run.
The require_once() statement includes and runs the specified file during script execution. This behavior is similar to the require() statement, with the only difference being that if the code in the file has been included, it will not be included again.
The include_once() statement includes and runs the specified file during script execution. This behavior is similar to the include() statement, with the only difference being that if the code in the file has been included, it will not be included again. As the name of this statement implies, it will only be included once.
The () function will read the specified file into and execute the program inside.
For example: include('/home/me/myfile');
The program code in the imported archive will be executed, and when executed, these programs will have the same variable scope as the variable scope in the source file that calls the include() function. You can import static files from the same server, or even import files from other servers by combining the include() and fopen() functions.
2. The function of the include_once() function is almost the same as include()
The only difference is that the include_once() function will first check whether the file to be imported has been imported in other places in the program. If there is one, the file will not be imported again (this function is sometimes very important. For example, the file to be imported declares some functions that you have defined by yourself. If you repeatedly import this file in the same program, an error message will occur during the second import, because PHP does not allow functions with the same name to be repeatedly declared for the second time).
The () function will read the content of the target file and replace it with these read contents.
This read-in and substitution action happens when the PHP engine compiles your program code, not when the PHP engine starts to execute compiled program code (the working method of the PHP 3.0 engine is to compile one line and execute one line, but it changes when PHP 4.0. PHP 4.0 first compiles the entire program code, and then executes the compiled program code at one time, and no program code will be executed during the compilation process). require() is usually used to import static content, while include() is suitable for importing dynamic program code.
4. Just like the include_once() function, the require_once() function will first check whether the content of the target file has been imported before. If so, the same content will not be imported again.