How to capitalize first letter of word in php without any function

I am trying to capitalize the first letter of word in php without using any function But i am not able do it , but i am struggling with this. Please tell me its answer.

<?php

   $str ="the resources of earth make life possible on it";
   $str[0] = chr(ord($str[0])-32);  
   $length = strlen($str);
  
   for($pos=0; $pos<$length; $pos++){
           if($str[$pos]==' '){
               $str[$pos+1] = chr(ord($str[$pos+1])-32);
       }
    }
   echo $str;
?>