How to make button readonly?

I need to make a button readonly or not clickable...i was informed on the following but it only works for textfields not buttons. Anyone??

echo '<INPUT TYPE=button VALUE="Modify" READONLY="readonly" NAME="modifybutton" />';

Code (markup):

 

Try using the following:

<INPUT TYPE=button VALUE="Modify" disabled="disabled" NAME="modifybutton" />

 

Thank you!