submitBtn.addEventListener('click', function(){
   var phpRequest = new XMLHttpRequest();
   phpRequest.open('POST', 'form.php');
   phpRequest.onload = function(){
       var errorMessages = JSON.parse(phpRequest.responseText);
       var errorVars = [];
       for(i = 0; i < Object.keys(errorMessages).length; i++) {
          var objValue = errorMessages[Object.keys(errorMessages)[i]];
          errorVars.push(objValue)
       }
   }
   var form = document.getElementById('myForm'); // Cambia este valor por el identificador de tu formulario (atributo id)
   phpRequest.send(new FormData(form));
});