How can developers avoid common mistakes, such as not properly checking form input before processing it in PHP scripts?

Developers can avoid common mistakes like not properly checking form input before processing it in PHP scripts by implementing server-side validation. This involves validating user input for data type, length, format, and any other specific requirements before processing it. By validating input before processing, developers can prevent security vulnerabilities such as SQL injection, cross-site scripting, and other forms of attacks. 

// Example of validating form input before processing

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $username = $_POST["username"];
    $password = $_POST["password"];

    // Validate input
    if (empty($username) || empty($password)) {
        echo "Username and password are required.";
    } else {
        // Process the input
        // Code to process the input goes here
    }
}

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