Inserting data to table (mysqli insert)

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)");

Note: Add backticks ` for column names in your insert query as some of your column names are reserved words.

In mysqli_query(first parameter should be connection,your sql statement) so

$connetion_name=mysqli_connect("localhost","root","","web_table") or die(mysqli_error());
mysqli_query($connection_name,'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)');

but best practice is

$connetion_name=mysqli_connect("localhost","root","","web_table") or die(mysqli_error());
$sql_statement="INSERT INTO web_formitem (ID, formID, caption, key, sortorder, type, enabled, mandatory, data) VALUES (105, 7, Tip izdelka (6), producttype_6, 42, 5, 1, 0, 0)";
mysqli_query($connection_name,$sql_statement);