Define a user like this:
GRANT ALL ON *.* TO 'myuser'@'localhost' IDENTIFIED BY 'super$ecret';
Then try a PHP script like this:
mysqli_connect("localhost", "myuser", "super$ecret");
What happens and why? How could you avoid it?
And for glory, what single line could you add that would prevent the error, without making any changes to the mysqli_connect line?