I found a website with the error below:
Notice: Undefined index: welcome in /var/www/html/blah/index.php on line 14
Is that a vulnerability?
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A bug like that could be a vulnerability, but it does not have to be one. It might not even be likely that it is one. While all vulnerabilities are bugs, all bugs are not vulnerabilities. It is impossible to say more without more information.
The error message is simply what you get when you try to look up a key that does not exist in a PHP associative array. So that is the error you would get if you run
$my_array["welcome"] when there is no value for