在替换文本中的内容时无限循环[重复]

I'm new to PHP and try to implement some stuff on my own without using all built-in PHP functions. Here I want to simply search and replace stuff in my text as str_replace() does it. Except I try to do it without that function.

Right now I have this code:

<?php

$offset = 0;

parse_str(file_get_contents("php://input"), $_POST);
if (isset($_POST['text'])&&isset($_POST['search'])&&isset($_POST['replace'])) {
     $text = $_POST['text'];
     $search = $_POST['search'];
     $replace = $_POST['replace'];
     $search_len = strlen($search);
     $strpos = strpos($text,$search,$offset);
        while ($strpos<=strlen($text)){
            $offset = $strpos + $search_len;
            $text = substr_replace($text,$replace,$strpos,$search_len);//line 15
            $strpos = strpos($text,$search,$offset);
 }
  echo $text;

 } ?>

<hr>
<form action="new.php" method="POST" name="form">
  <textarea name="text" rows="6" cols="30" ></textarea><br><br>
  search  <br><br>
  <input type="text" name="search" ><br><br>
  replace<br><br>
  <input type="text" name="replace" ><br><br>
  <input type="submit" value="Submit"><br><br>
</form>

But for some reason my while loop falls into an infinite loop and the scripts ends with an error.

Error:

Fatal error: Maximum execution time of 30 seconds exceeded in C:\Users\Piyush\PhpstormProjects\untitled ew.php on line 15

I fail to see why, where and how my while loop falls into an infinite loop?

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