There is no function named main() except in the PHP source. In PHP 4.3.0, a new type of error handling in the PHP source (php_error_docref) was introduced. One feature is to provide links to a manual page in PHP error messages when the PHP directives html_errors (on by default) and docref_root (on by default until PHP 4.3.2) are set.
Sometimes error messages refer to a manual page for the function main() which is why this page exists. Please add a user comment below that mentions what PHP function caused the error that linked to main() and it will be fixed and properly documented.