How to set an input box that will only accept numeric values?

You'll need to use some Javascript, unless HTML5 is supported.

Use clients side and server side techniques.

First, you can use HTML5 input type number to restrict only number entries:

 <input type="number" name="txtBox1" />

This will work only in browsers that support this HTML5 input type.

Next is JavaScript...

 function isNumber(e) {
 var char = (e.which) ? e.which : event.keyCode
 if (char > 31 && (char < 48 || char > 57))
    return false;
 return true;
 }

 <input type="text" name="number" onkeypress="return isNumber(event)">

Server side--PHP

 if (!is_numeric($num)) {
     echo 'Input is not numeric';
 }

thanks..but my browser does not support html5 so let me try the js

by the way, in addtion, in the input box i need to input the year. a year is only 4 digits but i can type more than 4 digits. is there anyway to limit the number of digits or characters to be entered to 4?

Add the maxlength attribute to the input element.