Replace the Nth occurrence of characters in a string without using regular expressions

For example, there is a string: $a='hello world hello pig hello cat hello dog hello small boy';
Then I want to change the hello that appears for the third time to good-bye, for example:
'hello world hello pig good-bye cat hello dog hello small boy';
In this case, I couldn’t find PHP’s built-in function for a while, and I wrote this simple little function when I was required not to use regular expressions. If you have any recommendations for good built-in functions, please leave a message:)
Reprinted from PHP interview questions: http://phpmst.com/

 /*
 * $text is the input text;
 * $word is the original string;
 * $cword is the string that needs to be replaced;
 * $pos refers to $word The position of the Nth occurrence in $text, starting from 1
 * */
 function changeNstr($text,$word,$cword,$pos=1){
 $text_array=explode($word,$text );
 $num=count($text_array)-1;
 if($pos>$num){
 return "the number is too big!or can not find the $word";
 }
 $result_str='' ;
 for($i=0;$i<=$num;$i++){
 if($i==$pos-1){
 $result_str.=$text_array[$i].$cword;
 } else{
 $result_str.=$text_array[$i].$word;}

 }

 return rtrim($result_str,$word);
 }
$text='hello world hello pig hello cat hello dog hello small boy';
$word='hello';
$cword='good-bye';
echo changeNstr($text,$word,$cword,3);
//Output: hello world hello pig good-bye cat hello dog hello small boy
?>