Running multiple SQL queries is not working

I am trying to run multiple queries in 1 php:

$sql = "UPDATE table1 SET column='1' WHERE id='1'";
$sql.= "INSERT INTO table2(column1, column2) VALUES ('1', '2')";
if (!$mysqli->multi_query($link, $sql)) {

When I run this I get a error. Does anybody know what is wrong with my script?

Here is my full script:

<?php
$link = mysqli_connect("localhost", "root", "", "db");                   
if($link === false){
die("ERROR: Could not connect. " . mysqli_connect_error());
}
$sql = "UPDATE table1 SET column='1' WHERE id='1'";
$sql.= "INSERT INTO table2(column1, column2) VALUES ('1', '2')";
if (!$mysqli->multi_query($link, $sql)) {

echo "Success";
} else {
 echo "ERROR: Could not able to execute $sql. " . mysqli_error($link);
}

// Close connection
mysqli_close($link);
?>

Edit 1:

I get the following error: Notice: Undefined variable: mysqli in ......../script.php on line 24

Fatal error: Call to a member function multi_query() on a non-object in ......../script.php on line 24