How to validate the username textfield in a form in php - php

Question

I want to check in the username text field to be typed only letters how can i do it in PHP? Plus in the password area to input both letters and numbers. When I click the Register button it displays Successful Registration although the form is not filled and I also put an else statement to display a message to fill all the fields.
the form
<form action = "register.php" method = "POST">
Username: <input type="text" name="username" > <br /><br/>
Password: <input type="password" name="password"> <br /><br/>
Confirm Password: <input type="password" name="repassword"> <br /><br/>
Type:
<select name="type">
<option value="Choose">Please select..</option>
<?php
$sql=mysql_query("SELECT type FROM type");
while($row=mysql_fetch_array($sql)){
echo "<option value='".$row['type']."'>".$row['type']."</option>";
}
?>
</select><br/><br/>
<input type="submit" value="Register" name="submit">
</form>
the register code
<?php
require('connect.php');
$username=$_POST['username'];
$password=$_POST['password'];
$repass=$_POST['repassword'];
$type=$_POST['type'];
if (isset($_POST['submit'])){
//input only letters in username textfield
if (isset($_POST['username']) && isset($_POST['password']) && isset($_POST['type'])){
$sql = "INSERT INTO users (username, password, type) VALUES ('$username', '$password', '$type')";
$result = mysql_query($sql);
echo "Successful Registration";
}
if(!$result){
$msg = "User Failed to be Registered.";
}
}
else{
echo "Please fill all the fields.";
}
?>

Answer 1

don't validate the userinput with the isset() function. Instead use !empty().
Because isset() also returns true if the POST variable is an empty string.
Also be careful with your SQL-Query (SQL-Injection), you have to escape the $_POST inputs.
You can use this code
$username = real_escape_string ($_POST['username']);
$password = real_escape_string ($_POST['password']);
$type = real_escape_string($_POST['type']); //if type is int, use intval() to escape
BTW: you don't validate the password against the repassword.