PHP Guard Clause

guard-class-test.php

<?php

class TestClass
{
    public function isUsernameStartsWithLetter($username)
    {
        // if empty string trow exception
        if (empty($username)) {
            throw new Exception('Username is empty');
        }

        return preg_match('/^[a-zA-Z]/', $username);
    }
}

$obj = new TestClass();
var_dump($obj->isUsernameStartsWithLetter('')); // exception