Include vs Include_Once and Require vs Require_Once - PHP

Include()

The include function simply includes a file, and that is all. If the file isn't found a warning is raised, but the script will continue to execute.

Require()

Require has very similar functionality to include(). However, if a file is not found a warning will be raised and the script will halt completely.

Include_Once() and Require_Once()

You should use these functions if you do not want to include a file multiple times (which can cause errors with functions and classes).

e.g.

file1.php includes file2.php and file3.php.
file2.php also includes file3.php

Without the _once() functions file3 will be included twice, but with the _once() function it will only be included once.

source : http://www.nusuni.com/blog/2007/02/14/php-tip-include-vs-include_once-and-require-vs-require_once/