random username checker

check_user.php

<?php
$dbh = ['user_10', 'user_02', 'user_05', 'user_08', 'user_07', 'user_06', 'user_09'];
$a = function  ($un = '') use ($dbh, &$a){
    $random_digit = sprintf("%02d", rand(0, 10));
    $username_random = "user_" . $random_digit;
    if(strlen($un)){
        $username = $un;
    }else{
        $username = $username_random;
    }
    if(in_array($username, $dbh)){
        //  username exists, check again
        echo $username . ' exists  ';
        return $a();
    }else{
        //  username does not exist, use it
        echo '  use $username ';
        return $username;
    }
};
$b = $a();
echo $b;