Need to validate form by PHP.

[shakti]

I have made a form which have two input field.
Now I need to validate those input.
Here is the Form

<form name="form1" action="form2.php" method="post">
<input type="text" value="" name="credit" size="11" maxlength="11"/>
<input type="text" value="" name="credit2" size="11" maxlength="11"/>
<input type="submit" value="submit" />
</form>

Now I need to validate them.
As I am only 16 and completely new in PHP, I am unable to validate them.
Please give me some codes which can validate those field.
I need to set the values of those fields as-


1. Values must be digits(0123456789).
2. Maximum and minimum length is 11.
3. 2 Values must be same.
4. First 3 digit must be 017 or 018 or 019 or 011.
5. If first 3 digit is 017 then $provider = "GrameenPhone";
6. If first 3 digit is 018 then $provider = "Aktel";
7. If first 3 digit is 019 then $provider = "Banglalink";
8. If first 3 digit is 011 then $provider = "Citycell";

Please do them with php.
Make it by -
if ($_POST['credit'] != $_POST['credit2']) {
echo "Valuse are not same";
}
elseif (Please help me to code here to validate values as disired.) {
}
else {
I will handle this section.
}
Please help me by doing this.

[Nile]

<?php
  $digits = "234234929";
  switch (substr($digits, 0, 3)) {
      case '123':
          $provider = "Hello";
          break;
      case '234':
          $provider = "Hey";
          break;
      default:
          $provider = "Hi";
  }
  echo $provider;
?>

[prasanthmj]

Checkout the PHP form validation script here:
PHP form validation

Also, it is a good idea to do the validations at the client side as well, using JavaScript(the response is immediate)
checkout this JavaScript form validation script.