Código PHP:
<script>
function activar {
if(radiobutton1=="on")
{
button.disabled=false;
}
if(radiobutton2=="off")
{
button.enabled=true;
}
}
</script>
<form name="form1" method="post" action="">
<p>
<input name="radiobutton1" type="radio" value="radiobutton">
activar<br>
<input name="radiobutton2" type="radio" value="radiobutton">
desactivar
</p>
<p>
<input type="submit" name="button" value="Mi boton">
</p>
</form>