PHP provides four functions which enable you to insert code from other files.
All four can take a local file or URL as input. None of them can import a remote file. require() and include() functions are virtually similar in their function except for the way they handle an irretrievable resource. include() and include_once() provide a warning if the resource cannot be retrieved and try to continue execution of the program if possible. require() and require_once functions provide stop processing the page if they cannot retrieve the resource.
Why include_once() and require_once()
The include_once() and require_once() functions are handy in situations where multiple files may reference the same included code. For example:
File A.php includes File B.php and C.php
File B.php includes File C.php
File C.php has been included twice, so the interpreter would print an error. Since a function cannot be redefined once it’s declared, this restriction can help prevent errors.
If both File A.php and File B.php use include_once() or require_once() to import File C.php, no errors would be generated. PHP would understand that you only want one instance of the code in File C and would not try to redeclare the functions.
It is best to use require_once() to include files which contain necessary code and include_once() to include files that contain content which the program can run without e.g. HTML, CSS, etc.