A prime number program

Hello friends,
please dont feel disappointed by my question. Im not too perfect in php.
Im battling at writing a prime number program. I did it to this point: if the textbox is empty, it tells the user to input a number. When a number less than 2 is entered, it tells the user to input a higher value. Now my problem is, when i input a greater than 2 number like 4, it still tells me its a prime number. Please help me.
Thanks,
Golden Waters.
 
Ok DesignerBill. Here it is:
<?php
//collects the number entered by user.
$num = $_POST['num'];
if(empty($num)){
die("ERROR: Please enter a number.");
}
if($num<2){
echo "This is no prime number.";
}elseif($num %= 1 && ...){\\i have problems here completing this code.
echo "This is a prime number";
}else{
echo "This is NOT a prime number";
}

?>
 
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