Find and replace-PHP

<?php
$offset = 0;
if(isset($_POST['text'])&&isset($_POST['searchfor'])&&isset($_POST['replacewith'])){
 $text = $_POST['text'];
 $search = $_POST['searchfor'];
 $replace = $_POST['replacewith'];

 $search_length = strlen($search);
if(!empty($text)&&!empty($search)&&!empty($replace)){
 while($strpos = strpos($text, $search, $offset)) {
  $offset = $strpos + $search_length;
  $text = substr_replace($text,$replace, $strpos, $search_length);
}
echo $text;
}else{
echo 'Please fill in all fiels';
}
}
?>
<form action="index.php" method="POST">
<textarea name="text" rows="6" cols="30"></textarea><br><br>
Search for:<br>
<input type="text" name="searchfor"><br><br>
Replace with:<br>
<input type="text" name="replacewith"><br><br>
<input type="submit" value="Find and Replace"><br><br>
</form>