ช่วยเช็คหน่อยค่ะ ลองมาหลายทีแล้ว ยังไม่ได้เรย validation ด้วย php

<?php

mysql_connect("localhost","root","");
	mysql_select_db("my_db");

// define variables and set to empty values
$usernameErr = $passwordErr= $emailErr = $postcodeErr = "";
$username = $password = $email = $postcode = "";

if (isset($_POST['submit'])) 
{
 	$flag= FALSE;
	if($flag) {
 header("location: FormPHPValidateInsert.php");
exit();
}
	///////////////////////
   if (empty($_POST["username"]))
     {$usernameErr = "Username is required";
	 $flag= TRUE;}
   else
     {
     $username = test_input($_POST["username"]);
     // check if name only contains letters and whitespace
     if (!preg_match("/^[a-zA-Z ]*$/",$username))
       {
       $usernameErr = "Only letters and white space allowed";
	   $flag= TRUE; 
       }
     }

   if (empty($_POST["password"]))
     {$passwordErr = "Password is required";
	 $flag= TRUE;}
   else
     {$password = test_input($_POST["password"]);
	 }

 
   if (empty($_POST["email"]))
     {$emailErr = "Email is required";
	 $flag= TRUE;}
   else
     {
     $email = test_input($_POST["email"]);
     // check if e-mail address syntax is valid
     if (!preg_match("/([\w\-]+\@[\w\-]+\.[\w\-]+)/",$email))
       {
       $emailErr = "Invalid email format";
	   $flag= TRUE; 
       }
     }

   if (empty($_POST["postcode"]))
     {$postcodeErr = "Postcode is required";
	 $flag= TRUE;}
   else
     {$postcode = test_input($_POST["postcode"]);}
}

function test_input($data)
{
     $data = trim($data);
     $data = stripslashes($data);
     $data = htmlspecialchars($data);
     return $data;
}
?>

<h2>PHP Form Validation Example</h2>
<p><span class="error">* required field.</span></p>
<form method="post" action="<?php $_SERVER["PHP_SELF"];?>"> 
   <label>Username:</label> <input type="text" name="username" value="<?php echo $username;?>"> <span class="error">* <?

 echo $usernameErr;?></span>
   <br><br>
   <label>Password:</label> <input type="text" name="password" value="<?php echo $password;?>"> <span class="error">* <?

echo $passwordErr;?></span>
   <br><br>
   <label>Email:</label> <input type="text" name="email" value="<?php echo $email;?>"> <span class="error"><?php echo 

$emailErr;?></span>
   <br><br>
   <label>Postcode:</label> <input type="text" name="postcode" value="<?php echo $postcode;?>"> <span class="error"><?php 

echo $postcodeErr;?></span>
   <br><br>
   <input type="submit" name="submit" value="Submit"> 
</form>