How do I display a MySQL error in PHP for a long query that depends on the user input?

To display a MySQL error in PHP for a long query that depends on user input, you can use the mysqli_error() function. This function returns a string description of the last error.

Here's an example of how you can use it:

<?php

$query = "SELECT * FROM users WHERE username='$username' AND password='$password'";

$result = mysqli_query($conn, $query);

if (!$result) {
  // query failed
  $error = mysqli_error($conn);
  echo "Error: $error";
} else {
  // query was successful
  // do something with the data
}

In this example, $conn is a connection to the MySQL database that was created using mysqli_connect(). If the query fails, the mysqli_error() function will return a string describing the error, which can then be displayed to the user.

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It's important to note that you should not display raw MySQL errors to the user, as they may contain sensitive information. Instead, you should display a generic error message to the user, and log the full MySQL error message for debugging purposes.