An extremely simplified take on the latter part of my statement:
<?php
if (!$_POST) {
?><form method='post' action='<?=$_SERVER['PHP_SELF']; ?>'>
URL to visit: <input type='text' name='url'>
<br>
<input type='submit' name='submit' value='Go!'>
</form>
<?php
}
else {
header("Location: $_POST['url']");
exit;
}