How can a PHP script validate user input to ensure only numbers between 0-9 are entered and display an error message if letters are inputted?

To validate user input in PHP to ensure only numbers between 0-9 are entered, you can use regular expressions to check if the input contains only numeric characters. If letters are inputted, you can display an error message to prompt the user to enter valid input.

$input = $_POST['input']; // Assuming input is received via POST method

if (preg_match('/^[0-9]+$/', $input)) {
    // Input contains only numbers between 0-9
    echo "Input is valid.";
} else {
    // Input contains letters or other characters
    echo "Error: Please enter only numbers between 0-9.";
}