: How to limit the submission of a page

For help: how do I limit the submission of a page? I want to implement this function. the target page determines the ajax submission of the source page and whether it is a source page from a limited number, such as log_in.php. if not, so what should I do in the/directory ?, HTMLcode $. ajax ({u for help: how do I limit the submission of a page?
I want to implement this function,

Ajax submission of the source page for determining the target page,

And determine whether it is from a limited source page, such as log_in.php,

If not, go to the/directory.

How?

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HTML code

$. Ajax ({url: your-url, dataType: "html", type: 'post', data: 'Something = 1', success: function (data) {...}});
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$ _ SERVER ['http _ referer']