PHP validation bug?

Hello,
I have interesting problem when I trying to validate value. The code is valid and works white-out any errors, however result is wrong.
Code:

<?php
$s='12345';
$i=false;
if(isset($s)&&($s+=0)>0&&isset($s[4])&&!isset($s[5])){$i=true;}//$i must be true, but its not!
?>

PHP:

$i must be true, but its false.

If we separate validation we will get true in all tests:

<?php
$i='';
$s='12345';
if(isset($s)){$i.=1;}
$s='12345';
if(($s+=0)>0){$i.=1;}
$s='12345';
if(isset($s[4])){$i.=1;}
$s='12345';
if(!isset($s[5])){$i.=1;}
echo $i;
?>

PHP:

outputs 1111
Is this happens only for me?
Thanks.

 

if($s && ($s+=0)>0 && strlen($s)==5 && strlen($s)<6){
$i = 1;
}

PHP:

This seems to be working

 

yea works for me to, but I need code what I posted to work...

 

What do you mean? =/

if($s && ($s+=0)>0 && strlen($s)==5 && strlen($s)<6){
$i = true;
}

PHP:

 

I mean what this code is 100% valid, but it doesn't work:

if(isset($s)&&($s+=0)>0&&isset($s[4])&&!isset($s[5])){$i=true;}

PHP: