What are some common mistakes when trying to make variables changeable by users in PHP scripts?

One common mistake when trying to make variables changeable by users in PHP scripts is not properly sanitizing user input, which can lead to security vulnerabilities such as SQL injection or cross-site scripting attacks. To solve this issue, always validate and sanitize user input before using it in your script. Another mistake is not properly checking if the user input is set before assigning it to a variable, which can lead to undefined variable errors. Ensure that you check if the user input is set before assigning it to a variable to avoid these errors.

// Example of properly sanitizing user input and checking if it is set before assigning it to a variable

// Sanitize user input using filter_input
$userInput = filter_input(INPUT_POST, 'user_input', FILTER_SANITIZE_STRING);

// Check if the user input is set before assigning it to a variable
if(isset($userInput)) {
    // Use the sanitized user input in your script
    echo "User input: " . $userInput;
} else {
    echo "User input is not set";
}