How to return string that extended by recursive loop in a PHP function?

How to return string that extended by recursive loop in a php function?

<?php

function test_loop($string_name,$x)
{
    $x = $x + 1;
    if($x < 10)
    {
        $string_name = $string_name . $x . "##";
        test_loop($string_name,$x);
    }
    return $string_name;
}

$bababa = test_loop(0,1);

echo $bababa;

//output is 02##


?>
How to make variable $bababa store value 02##3##4##5##6##7##8##9## ?