To prevent SQL injection attacks when validating user input in PHP, you can use parameterized queries or prepared statements to interact with your database. This involves using placeholders in your SQL statements to represent user input, and then binding the actual input values to these placeholders at runtime.

For example, instead of concatenating user input directly into a SQL statement like this:
``` 
$username = $_POST['username'];
$password = $_POST['password'];

$query = "SELECT * FROM users WHERE username = '{$username}' AND password = '{$password}'";
``` 
You can use prepared statements with placeholders like this:
``` 
$username = $_POST['username'];
$password = $_POST['password'];

$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username AND password = :password');
$stmt->execute(['username' => $username, 'password' => $password]);
``` 
In this example, the :username and :password are placeholders that represent the user input values. The execute() method then binds the actual values to these placeholders at runtime, ensuring that the user input is properly escaped and protected against SQL injection attacks.

Using prepared statements and parameterized queries is considered a best practice for preventing SQL injection attacks and should always be used when interacting with user input in PHP.
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