MySQLi query to loop through array and update multiple rows

I have an array like:

$postdata[1] = 'This';
$postdata[2] = 'That';
$postdata[3] = 'The other';

And I want to loop through the array and update all of the rows where ID corresponds to the array key. Like:

foreach ($postdata as $key => $value) {
  if ($key == 1) {
    $update = $db->query("UPDATE site_email_templates SET Content='$postdata[1]' WHERE ID = 1");
  } else if ($key == 2) {
    $update = $db->query("UPDATE site_email_templates SET Content='$postdata[2]' WHERE ID = 2");
  } else if ($key == 3) {
    $update = $db->query("UPDATE site_email_templates SET Content='$postdata[3]' WHERE ID = 3");
  }
}

What would be the simplest way to do this, not particularly knowing how many array keys there are, and keeping it all in one query?

Answer

Note: My answer is based on the PDO driver which in many aspects is better than mysqli. If you need mysqli solution please check the other answer provided by @Your Common Sense

The code below is tested on real environment and served with prepared statement preventing SQL-injection:

$sql = "UPDATE `site_email_templates` SET `Content` = (:content) WHERE `Id` = (:id)";
$stmt = $dbConn->prepare($sql);
foreach ($postdata as $id => $content)
{
    $stmt->execute([':id' => $id, ':content' => $content]);
}

For more details about SQL injection you can read more:
https://www.owasp.org/index.php/SQL_Injection