<?php
function wordlength($txt, $limit)
{
   $words = explode(' ', $txt);

   foreach($words as $v)
   {
       if(strlen($v) > $limit)
       {
            return false;
       }
   }

   return true;
}
?>

Uses like this:
<?php

$txt = "Onelongword and some small ones";

if(!wordlength($txt, 10))
{
    die("One of the words where too long");
}