<input type="number" value="" name="fact">
<?php
$fact = ["fact"];
?>
<form method="POST">
<input type="number" value="123456789" name="fact">
<input type="submit">
</form>
<?php
$fact = isset($_POST["fact"]) ? $_POST["fact"] : '';
echo $fact;
?>
$ php -S 127.0.0.1:8080
<?php
$foo = 'bar'; // переменная
?>