What are the best practices for handling user input in PHP to avoid errors like "Undefined Constant"?

When handling user input in PHP, it is important to sanitize and validate the input to prevent errors like "Undefined Constant". One way to avoid this error is to check if the input exists using isset() before using it in your code. This ensures that the input is defined before trying to access its value. 

// Check if the user input is set before using it
if(isset($_POST['user_input'])){
    $user_input = $_POST['user_input'];
    // Use the user input in your code
    echo "User input: " . $user_input;
} else {
    echo "User input is not set";
}

Keywords

user input PHP undefined constant error handling validation

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