First, you're going to get a parse error from the missing semicolon after this line:
$input .= " from php"
Second, you must define a string variable before using the concat assignment operator like that.
Third, you should use the $_GET superglobal to retrieve variables sent through GET.
$input = $_GET['input'];
$input .= " from php";
$output = "output=$input";
Should work, then : )