<?php

function strtolowertr($metin){ 
    return mb_convert_case(str_replace('I','ı',$metin), MB_CASE_LOWER, "UTF-8"); 
} 
function strtouppertr($metin){ 
    return mb_convert_case(str_replace('i','İ',$metin), MB_CASE_UPPER, "UTF-8"); 
} 
function ucfirsttr($metin) { 
    $metin = in_array(crc32($metin[0]),array(1309403428, -797999993, 957143474)) ? array(strtouppertr(substr($metin,0,2)),substr($metin,2)) : array(strtouppertr($metin[0]),substr($metin,1)); 
return $metin[0].$metin[1]; 
} 

$s = "this is the, sample text.for replace the each words in, this'text sample' words";
echo preg_replace_callback('~\b\w+~u', function ($m) { return ucfirsttr($m[0]); }, $s);