<?php
if (isset($_POST['myText'])&&!empty($_POST['myText'])&&isset($_POST['strFind'])&&!empty($_POST['strFind'])&&isset($_POST['strReplace'])&&!empty($_POST['strReplace'])) {
    $myText=$_POST['myText'];
    $strFind=$_POST['strFind'];
    $strReplace=$_POST['strReplace'];
    $offset=0;
    $myText = str_replace($strFind, $strReplace, $myText);
    echo $myText;
}
?>
<hr>
<form action="index.php" method="POST">
    <textarea name="myText" rows="6" cols="30"></textarea><br><br>
    <strong>String to find</strong><br>
    <input type="text" name="strFind"><br><br>
    <strong>String to replace</strong><br>
    <input type="text" name="strReplace"><br>
    <input type="submit" name="submit">
</form>