$str = str_replace("ll", "", "good golly miss molly!", $count);
echo $count; // 2

function str_replace_once($search, $replace, $text){
   $pos = strpos($text, $search);
   return $pos!==false ? substr_replace($text, $replace, $pos, strlen($search)) : $text;
}

$str = 'Helo World!';
$str = str_replace_once('l', 'll', $str);
// $str = 'Hello World!';

<?php
function replacment($search, $replace, $text, $c){
    if($c > substr_count($text, $search)){
       return false;
   }
   else{
       $arr = explode($search, $text);
       $result = '';
       $k = 1;
        foreach($arr as $value){
            $k == $c ? $result .= $value.$replace : $result .= $value.$search;
            $k++;
        }
        $pos = strripos($result,$search);
        $result = substr_replace($result,'', $pos, $pos + 3);
        return $result;
   };
}
$st = 'Helo World and cool gold!';
$str = replacment('l', 'bb', $st,'2');
echo $str;
// $str = 'Helo Worbbd and cool gold!';
?>

$s='hello world, hello';
$s=implode('i',explode('ello',$s,2));
// hi world, hello

$s='hello torld, test';
if($p=strpos($s,'t')){
    $s[$p]='w';
}
// hello world, test