Custom PHP Validation Logic

<?php
function validateAge($age) {
    // Check if the age is not empty
    if (empty($age)) {
        return "Age is required.";
    }

    // Check if the age is a positive integer
    if (!ctype_digit($age) || $age <= 0) {
        return "Age must be a positive integer.";
    }

    // Check if the age is within a specific range
    if ($age < 18 || $age > 99) {
        return "Age must be between 18 and 99.";
    }

    // If all validation passes, return true
    return true;
}

// Example usage
$age = $_POST['age'];

// Validate the age
$validationResult = validateAge($age);

// Check the validation result
if ($validationResult === true) {
    // Age is valid, proceed with further actions
    echo "Age is valid.";
} else {
    // Age is invalid, display error message
    echo "Invalid age: " . $validationResult;
}
?>