Word Censoring-php

<?php
$find =array('alex','billy','dale');
$replace=array('a**x','b***y','d**e');

if(isset($_POST['user_input'])&& !empty($_POST['user_input'])){
$user_input = $_POST['user_input'];
$user_input_new=str_ireplace($find,$replace,$user_input);
echo $user_input_new;
}
?>
<hr>
<form action="index.php" method="POST">
<textarea name="user_input" rows="7" cols="20"><?php echo $user_input; ?></textarea><br><br>
<input type="submit" value="Submit">

</form>