What is the differene between require(), require_once(), include() and include_once()

• All these functions are used to include a file into another file.
• require() function includes a file and evaluates it. 
• require_once() includes the file if and only if is not added before.
• require_once() includes the file if and only if is not added before.
• require_once() is more recommended than require() function. For example, you want to include a file containing more functions and variable or variable initialization. It is better to use require_once().
• require() function produces a fatal error if the file is not found. whereas if you are using include function, It produces warning error.
• The difference between include() function and include_once() function is same as that of require() and require_once().
• include() function copies all the text from one file to the other file where the include() function is specified.
• include() function and require() functions are identical. But the difference in the case of how errors are showing.