How can PHP variables be properly concatenated with SQL queries to prevent errors?

When concatenating PHP variables with SQL queries, it is important to properly sanitize and escape the variables to prevent SQL injection attacks and syntax errors. One way to do this is by using prepared statements with parameterized queries. This allows variables to be safely inserted into the query without the risk of SQL injection. 

// Example of concatenating PHP variables with SQL queries using prepared statements

// Establish a database connection
$pdo = new PDO("mysql:host=localhost;dbname=mydatabase", "username", "password");

// Prepare the SQL query with a parameter
$stmt = $pdo->prepare("SELECT * FROM users WHERE username = :username");

// Bind the PHP variable to the parameter in the query
$username = $_POST['username'];
$stmt->bindParam(':username', $username);

// Execute the query
$stmt->execute();

// Fetch the results
$results = $stmt->fetchAll();

// Loop through the results
foreach ($results as $row) {
    echo $row['username'] . "<br>";
}

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