PHP - activar o crear un boton a partir de un checkbox

<script>

function checar(){

    if (boton.disabled == false){

        boton.disabled = true;

    }else{

        boton.disabled = false;

    }

}

</script>

<form>

<input type="checkbox" onchange="javascript:checar()" />

<input type="button" id="boton" value="Aceptar"/>

</form>