<?php

function valida_variavel_zero_um($valor){
     if((empty($valor)) OR (strlen($valor) != 1) OR ($valor != 1 && $valor != 0)){
        return "INVALIDO";
     }else{
        return $valor;
     }
}

$tests = [0, 1, false, true, "0", "1", [], "", 0.0, -0.0, 1.0];

foreach ($tests as $test)
{
    var_dump( valida_variavel_zero_um($test) );
}