密码中的错误消息在页面加载时可见？

How do i prevent the error-message from showing before the 'submit'-button is clicked?

For some reason the content of 'else'-statement is visible on pageload - the message "NO" should only appear after typing something else than 'a' in this.. what am i doing wrong?

<form method="POST">
  <input type="text" class="textfield" id="cursor" name="pass"> 
  <input type="submit" class="button" name="submit" value="OK"> 
</form>

<?php
$pass = $_POST['pass'];

    if($pass == 'a') {      
        echo "YES";
} 
    else{
        echo "NO";
}
?>

Add this PHP code:

<?php
if(isset($_POST['pass'])) {
    // your check here
}
?>

Test to see if the submit button is in the submitted data.

if (isset( $_POST['submit'] )) {

<form method="POST">
  <input type="text" class="textfield" id="cursor" name="pass"> 
  <input type="submit" class="button" name="submit" value="OK"> 
</form>

<?php
$pass = $_POST['pass'];
//You need to check if the form is submitted
    if($_POST['submit']){
        if(isset($pass) && !empty($pass) && $pass == 'a') {      
            echo "YES";
    } 
        else{
            echo "NO";
    }
?>