About the reason for the error in PHP input $_post[']

In PHP input $_post['] value when the page error, because the variable is not declared, so the page appears prompted undefined index, is because the first to use the isset to determine whether there is this variable.

such as: Isset ($_post['/* value */') to determine whether the variable is set.

If you need to determine the post, enter

if (Isset ($_post['/* value */')) && $_post['/* value */']== ')

About the reason for the error in PHP input $_post[']