What is the significance of ctype_digit() function in PHP validation and how can it be used effectively?

The ctype_digit() function in PHP is used to check if a string contains only digits. This function is useful for validating user input, such as checking if a phone number or an age is composed only of numbers. By using ctype_digit(), you can ensure that the input meets the required format before processing it further.

// Example of using ctype_digit() for input validation
$input = "12345";

if (ctype_digit($input)) {
    echo "Input contains only digits.";
} else {
    echo "Input contains non-digit characters.";
}