Allow URL fopen: If enabled, allow_url_fopen allows PHPs file functions - such as file_get_contents() and the include and require statements - can retrieve data from remote locations, like an
FTP or web site. Programmers frequently forget this and dont do proper input filtering when passing user-provided data to these functions, opening them up to code injection vulnerabilities. A large number of code injection vulnerabilities and hacks reported in PHP-based web applications are caused by the combination of enabling allow_url_fopen and bad input filtering. Off by default.