There's strrev()
to do this.
if you want to built it on your own (for learning?): your problem is that you're starting with your index one too high - a string of legth 25 is indexed from 0 to 24, so your loop hast to look like this:
for ($i = $len - 1; $i >=0;$i--)
{
echo $stringExp[$i];
}