Replace every instance of a substring in PHP

<?php
if(isset($_GET['submit']))
{
    if(strlen($_GET['text']) && strlen($_GET['search_for']) && strlen($_GET['replace_with']))
    {
        $text = $_GET['text'];
        $search_for = $_GET['search_for'];
        $replace_with = $_GET['replace_with'];
        $offset = 0;
        $search_for_length = strlen($search_for);
        $replace_with_length = strlen($replace_with);
        $text = ' '.$text; // that's how search position will never be 0
       
        while($search_for_position = strpos($text, $search_for, $offset))
        {
            $text = substr_replace($text, $replace_with, $search_for_position, $search_for_length);
            $offset = $search_for_position + $replace_with_length; // that's how $search_for will not be found in $replace_with when $search_for actualy exists in $replace_with
        }
       
        echo $text.'<hr/>';
    }
    else
    {
        echo 'Please fill up all the form input.<hr/>';
    }
}
?>

<form action="index.php" method="get">
    <textarea name="text" id="" cols="30" rows="5">Write something here.....</textarea><br/><br/>
    Search for:<br/>
    <input type="text" name="search_for" /><br/><br/>
    Replace with:<br/>
    <input type="text" name="replace_with" /><br/><br/>
    <input type="submit" name="submit" value="Search and Replace" />
</form>


<!-- Note: We can also create this program using str_replace function -->