Warning: Never ever refer to w3schools for learning purposes. They have so many mistakes in their tutorials.
According to the mysqli_query documentation, the first parameter must be a connection string:
$link = mysqli_connect("localhost","root","","web_table");
mysqli_query($link,"INSERT INTO web_formitem (`ID`, `formID`, `caption`, `key`, `sortorder`, `type`, `enabled`, `mandatory`, `data`)
VALUES (105, 7, 'Tip izdelka (6)', 'producttype_6', 42, 5, 1, 0, 0)")
or die(mysqli_error($link));
Note: Add backticks ` for column names in your insert query as some of your column names are reserved words.