php - How to check if a text box is not empty and the value is less than 500

I have a textbox that is an optional field for users to enter a number in to. If there is a number in there, I want to check to make sure it is less than 500 and then do something with it.

Here is what I am currently doing:

if($textbox!="" && <=500)
{
//action here
}

I tried replacing && with andif but still get an error Parse error: syntax error, unexpected T_IS_SMALLER_OR_EQUAL

What's the easiest way to do this?