How can I get an unknown username given an ID?

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That’s actually a very good question that has almost no good answers on Stack Overflow.

Basically you need the following steps to perform a SELECT query using mysqli:

  • create a correct SQL SELECT statement and replace all variables in the query with with question marks (called placeholders or parameters)
  • Prepare the resulting query
  • Bind all variables to the previously prepared statement
  • Execute the statement
  • get the mysqli result variable from the statement.
  • fetch your data

The detailed explanation can be found in my article, How to run a SELECT query using Mysqli, as well a helper function to simplify the routine.

Following this plan here is your code

$sql = "SELECT * FROM users WHERE id=?"; // SQL with parameters
$stmt = $conn->prepare($sql); 
$stmt->bind_param("s", $id);
$Stmt->execute();
$result = $stmt->get_result(); // get the mysqli result
$user = $result->fetch_assoc(); // fetch the data

now you can store the username in the session variable:

$_SESSION['name'] = $user['name'];

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