PHP if/else form - php

Question

I am trying to create a form where if you fill in a number like 11 you get a response with a color type. I know that I have to change $tulp, but I don't know what to do.
<form name= "gemiddelden" action= "" method="post">
<p>Geelzweem (%): <input type="text" name="getal1"/></p>
<p><input type="submit" name="Bepaal" value="Bepaal"/></p>
</form>
$tulp = date("H");
if($tulp == 10) {
echo("Lichtgeel");
}
else if($tulp >10 && $tulp <20)
{
echo("Geel");
}
else if($tulp >20 && $tulp <30)
{
echo("Zon geel");
}
else if($tulp >30 && $tulp <40)
{
echo("Diep Geel");
}
else if($tulp >40 && $tulp <50)
{
echo("Zwavel Geel");
}
else if($tulp >50 && $tulp <60)
{
echo("Intens Geel");
}
else if($tulp >60 && $tulp <70)
{
echo("Verzadigd Geel");
}
else if($tulp >70)
{
echo("Dit mag niet");
}
else {
echo("Error");

Answer 1

According to google translate Geelzweem represent the shade of yellow. Then the echoed texts are different shades. I am guessing that you need to display a color based on the input in the getal1 field. In that case you need to change the line
$tulp = date("H");
to
if (isset($_POST["getal1"])) {
$tulp = $_POST["getal1"];
} else {
// some default value here
$tulp = 20;
}
Also, although the standard says that it is not mandatory, it is best practice to specify the action of the form
<form name= "gemiddelden" action= "SOME URL HERE" method="post">

Answer 2

Needed to change $tulp to $tulp = $_POST['getal1'];