<form method=post>
<input type=input name=inputbox value=default>
<br>
<input type=submit>
</form>

<?php
$string = $_POST['inputbox'];
for ($i = 0; $i <strlen($string)/2;$i++)
{
echo $string[$i];
}
echo "<br>";
for ($i = strlen($string)/2+1; $i <strlen($string);$i++)
{
echo $string[$i];
}
?>