<?php

$str = 'hello kitty';
$pos = strpos($str, 'kit');
$word = extractWordAtPosition($str, $pos);
var_dump($word);

function extractWordAtPosition($str, $position)
{
    if ($position === false || !$str) {
        return '';
    }
    $words = explode(' ', $str);
    $characters = -1;
    foreach($words as $word){
        $characters += strlen($word);
        if($characters >= $position){
            return $word;
        }
    }
    return '';
}