User input in PHP

Im new to PHP and I am trying to read user input in my code but im getting the error: Use of undefined constant STDIN - assumed 'STDIN'

<?php 
    echo "<h1>Armstrong number</h1>";
    echo "Enter number: ";

   fscanf(STDIN, "%d", $input); //Error Line

    $n=$input;
    $sum=0;
    while($n != 0)   
   {   
       $rem = $n % 10;   
       $sum = $sum + $rem*$rem*$rem;   
       $n = $n / 10;   

   } 
    if ($input == $sum) 
    echo $input." entered is an armstrong no.";
    else
    echo $input." isnt an armstrong no.";
?>
``