How do I limit my ctype_alnum validation

Am Creating a login Script and i do not want users to put in number or alpha numeric characters in the username field and i use the the ctype_alnum function the problem is that the validation is also affecting the password field below is login script.

<div id="login">
<?php if(isset($_POST["login"])&&(empty($_POST["username"])||empty($_POST["password"])))
{
echo"<p class='style1' style='background:yellow;' >
Please You Have to Provide a Valid Username and password</p>";
$staff=false;}?>
<form action="<?php $_SERVER["PHP_SELF"] ?>" method="post">
<label style="font-style:bold;color:#FF9900;" ><h2>UserName:</h2> </label>
<?php if(isset($_POST["login"])&&(empty($_POST["username"])))
{
echo"Your User Name is Required";$staff=false;
}
elseif(ctype_alnum($_POST["username"])==true)
{
echo"You are allowed to put in Numbers<br>"; $staff=false;
}
?>
<input size="40" type="text" name="username" placeholder="UserName"/><br><br>
<label style="font-style:bold;color:#FF9900;" ><h2>Password:</h2> </label>
<?php if(isset($_POST["login"])&&(empty($_POST["password"])))
{
echo"Your Password is Required";$staff=false;
}			
?>
<input style="radius:6;" size="40" type="password" placeholder="Password" name="password"/><br></div>
<input type="submit" width="25px" value="Login" name="login"/>
</form>

so please help check the code for errors and how to limit the ctype_alnum validation to just the username field.