Paypal choose a donation and redirect to file

Question

I have the following code that allows the person to choose the donation amount and download the file. At the moment this allows the person to put in 0 and download. I want to make the minimum 0.99. How could I make it compulsory to make a donation otherwise don't download. I appreciate your help.

 <?php

echo "<p>Donate fixed amount to CharityName</p>
<form method='POST' action=''>
<select name='currency_code'>
<option value='EUR'>EUR</option>
<option value='GBP'>GBP</option>
<option value='USD'>USD</option>
<input type='text' name='donate_amount' value='0'>
<input type='submit' name='submit' value='Donate'></form>";

if(!empty($_POST['submit'])) {
// Form has been submitted
  if($_POST['donate_amount'] > 0) {
// Redirect to PayPal
header('Location: https://www.paypal.com/cgi-bin/webscr?cmd=_donations&item_name=Donation for XXX&amount='.$_POST['donate_amount'].'&currency_code='.$_POST['currency_code'].'&business=mypaypalemail@here.tld&cbt=Download the file&return=http://link-to-the-file&cancel_return=http://back-to-my-website');
}
else {
// No donation amount entered - proceed to file directly
header('Location: http://link-to-the-file/');
}
}

?>

Answer 1

See a sample:

if(!empty($_POST['submit'])) {
    if($_POST['donate_amount'] >= 0.99) {
        // Redirect to PayPal
    } else {
        exit('Please donate at least $.99');
    }
}

For Google analytics, I can't answer you, sorry.