Validate form before submit in php with the routes

i must validate a form before submit, in this code I use routes, so i can’t use $_SERVER[“PHP_SELF”] (maybe) someone can help me please?

<form name="user" action="index.php?action=save-report" method="post">
   <div class="form-group">
     <label for="report">Quante persone ci sono secondo te? </label>
     <br>
     <input type="number" id="report" name="report"  maxlength="180"  style="width: 5%" class="form-control" required >
   </div>
   <br>

   <button class="btn btn-primary">Invia</button>
   <input type="hidden" id="user__token" name="user[_token]" value="">
   <br>
</form>

Thank you

Answer

The best way to validate without refreshing is Javascript, just add a onchange attribute to the input and validate through js.

<input type="text" onchange="validate(this)" required>
<input type="submit" id="submit" value="Invia">

function validate(elem) {
   if (elem.value != "Ciao") {
      elem.classList.add("notValid"); //for example red border or red label...
      document.getElementById("submit").disabled = true;
   else {
      elem.classList.remove("notValid");
      document.getElementById("submit").disabled = false;
   }
}