well, im going to try to explain.. this is the official explanation of include() vs include_once() per php.net:
Quote:
The include_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include() statement, with the only difference being that if the code from a file has already been included, it will not be included again. As the name suggests, it will be included just once.
ok, what does this mean exactly? lets say for example, i have two files, once called functions.php. i also have header.php and i include functions.php, and another file called globals.php and i also include functions.php, if i do the include(), my php script will error out. i will get an error saying one of my functions has already been used, why? because i included the file functions.php twice, once in the header and again in globals.php. so to avoid getting an error, it would just be safe to use include_once() instead of include() function.
if you know you are messy with your php code, then its safer to use the include_once(), but if you keep track of all your code, then its ok to use the include() function.
hope i helped you explained the difrences between the include() and the include_once() functions in php
