Dealing with no input and setting a value for it in PHP

I have textboxes which a user is supposed to insert a numerical value. If the user misses a box the missed boxes value should be set as 0. How can I do this with a if statement? This is what I have tried but I can't seem to get it to work. When I miss a textbox is simply doesn't show "0" it's left blank.

$missed = 0;

if($usernumber == "")    { 
$usernumber = $missed;
}