PHP string validation function encoding fails

Question

The string "Árvíztűrő tükörfúrógép" seems to be invalid. Why?

if(isValid("Árvíztűrő tükörfúrógép",true,true))echo "VALID";else echo "INVALID";

function isValid($s, $abc=false, $accent=false, $numbers=false, $etc="")
{
    $valid="";

    if($abc)
        $valid = "abcdefghijklmnopqrstzuvwxyz";
    if($accent)
        $valid .= "öüóűőúéáí";
    if($numbers)
        $valid .= "0123456789";

    $valid .= $etc;

    for($i=0; $i < mb_strlen($s); $i++){
        $k = false;
        for($j = 0; $j < mb_strlen($valid); $j++){
            if(strtolower(mb_substr($s, $i, 1)) == mb_substr($valid, $j, 1))
                $k = true;
        }
        if(!$k) 
            return false;
    }
    return true;
}

Thanks for your help in advance!

UPDATE#1:

davebobak noticed that lowercase Á is not equal to á, but why?

Answer 1

strtolower() is not working for your Á character

á is NOT equal to strtolower("Á") in my test.

Remove your Á, and the space (like cars10 said) and it is valid.