Session-question about the unique PHP identifier. Do not understand, please explain

$ _ SESSION [& #39; uniqid & #39;] $ _ uniqidsha1 (uniqid (rand (), true); this $ _ SESSION [& #39; isn't uniqid & #39;] and $ _ uniqid necessarily equal .. Why can I check whether the value of $ _ SESSION [& #39; uniqid & #39;] is the same as that of $ _ uniqid to determine whether it is a normal registration. $ _ Uniqid is the value of the hidden field in the form sessionphp form

$ _ SESSION ['uniqid'] = $ _ uniqid = sha1 (uniqid (rand (), true )); isn't the $ _ SESSION ['uniqid'] and $ _ uniqid necessarily equal .. Why can I check whether the values of $ _ SESSION ['uniqid'] and $ _ uniqid are the same to determine whether they are registered normally.

$ _ Uniqid is the value of the hidden field in the form.