PHP, linking variables to a specific date in a form - php

Question

How can I assign a variable to a specific date for use in a form?
I want a warning to appear when the user tries to book too many a people on a specific date. I started writing this code but didn't know where to go from there:
<?php
$spaces = 20;
$num_people = GET('#people');
$message = "Unfortunately we don't have this many spaces avilable on this date. We have a maximum of $spaces.";
if($spaces < $num_people) {
echo $message;
}
?>
How could I assign the value $spaces to a specific date and link it into a form? Here is the form:
<form method="post">
Name:<br>
<textarea id="name"></textarea><br>
<br>
Date leaving:<br>
<br>
<textarea id="date"></textarea><br>
<br>
How many people?:<br>
<textarea id="people"></textarea><br>
<br>
<input type="submit">

Answer 1

$num_people = GET('#people');
This line should be
$num_people = $_GET['people'];
and
<textarea id="people"></textarea><br>
shoul be
<textarea id="people" name="people"></textarea><br>
And your method ahould be GET. Since you are sending GET request.
Your code should look like this:
<?php
if(isset($_GET['people'])){
$spaces = 20;
$num_people = $_GET['people'];
$message = "Unfortunately we don't have this many spaces avilable on this date. We have a maximum of $spaces.";
if($spaces < $num_people) {
echo $message;
}
}
?>
<form method="get" action="">
Name:<br>
<textarea id="name"></textarea><br>
<br>
Date leaving:<br>
<br>
<textarea id="date"></textarea><br>
<br>
How many people?:<br>
<textarea id="people" name="people"></textarea><br>
<br>
<input type="submit">