How does using single quotes versus double quotes impact the handling of PHP variables in SQL queries?

Using single quotes in SQL queries will treat variables as plain strings, whereas using double quotes will allow PHP variables to be interpreted and replaced with their values. To ensure proper handling of PHP variables in SQL queries, it is recommended to use double quotes when enclosing the query string.

// Example of using double quotes to properly handle PHP variables in SQL queries
$id = 1;
$query = "SELECT * FROM users WHERE id = $id";
$result = mysqli_query($connection, $query);