Trouble combing two different conditions for validation

[pgwodehouse]

I'm trying to create a donation script for a nonprofit's website, and I want to let the donors either pick a specific donation amount or enter their own amount. I have five radio buttons with a specific amount, and a sixth radio button labeled "other", and a textarea next to it for users to enter their own amount.

I want the script to validate this way:

Either:

a.) one of the five specific amount buttons is selected or
b.) both the "other" button is selected and the textfield contains an amount

I also want it to see errors and alert the users in two different ways:

a.) If no buttons are selected, "Please select a donation amount"
b.) If the "other" button is selected and the text field is empty, "Please enter a donation amount".

One of the main problems I'm encountering is that when none of the buttons are selected, both error message are appearing.

Any help would be greatly appreciated.

[devinemke]

<?php
if (!isset($_POST['submit']))
{
	echo '
	<form action="" method="POST">
	please choose amount:<br>
	<input type="radio" name="preset_amount" value="5">$5<br>
	<input type="radio" name="preset_amount" value="10">$10<br>
	<input type="radio" name="preset_amount" value="15">$15<br>
	<input type="radio" name="preset_amount" value="20">$20<br>
	<input type="radio" name="preset_amount" value="25">$25<br>
	<input type="radio" name="preset_amount" value="other">other $<input type="text" name="amount"><br>
	<input type="submit" name="submit" value="submit">
	</form>
	';
}
else
{
	if (!isset($_POST['preset_amount']))
	{
		echo 'Please select a donation amount';
	}
	elseif ($_POST['preset_amount'] == 'other' && !$_POST['amount'])
	{
		echo 'Please enter a donation amount';
	}
	else
	{
		if ($_POST['preset_amount'] == 'other') {$donation = $_POST['amount'];}
		else {$donation = $_POST['preset_amount'];}

	echo 'You choose to donate $' . $donation;
}
}
?>