Making a circumference calculator in PHP?

[lulz]

I haven't done PHP in about 3 months so my code is messed up and doesn't work.

<?php

echo '
<font face="verdana" size="1">
<b><u>Diameter</b></u>
<br /><br />
<form method="post" enctype="text/plain">
<b>Value</b>
<br />
<input type="text" value="number">
<br /><br />
<input type="submit" value="Submit" name="submit">
</form>
</font>
';

$number = $_POST['number'];

if($_POST['submit']==1) {
if($_POST['number']==1) {
if(is_numeric($number)) {

} else {
echo 'Only numbers are allowed';
}
$answer = $number*3.14;
echo 'The answers is '.$answer.'.';
} else {
echo 'Error 1';
}
} else {
echo 'Error 2';
}

?>