Non-alphanumeric characters in input fields!

Let's say $mystring came from something like this:

file1.
Input text field inside a form named mystring. Used form method: POST.

file2.
$mystring=$_POST['mystring'];
function rvs ($argument)
{
return ereg_replace("[^a-zA-Z0-9]", "", $argument);
}
$result= rvs ($mystring);
$a_1 = strlen($mystring);
$a_2 = strlen($result);
if($a_1 != $a_2)
{
echo "Error: Your input used non-alphanumeric characters!";
exit;
}

Look at this example for a better description of rvs() function:

function rvs ($argument)
{
return ereg_replace("[^a-zA-Z0-9]", "", $argument);
}
$example="Daniel - 20 years old - result 20%";
$result = rvs ($example);
echo $result;

This will print:
Daniel20yearsoldresult20

Please post comments with a better way to do this ;).