PHP - Update Data Where Id Exist , Insert If Not Exist Id

//database
create table mydata (
	id int(11) NOT NULL PRIMARY KEY AUTO_INCREMENT,
	fname varchar(20),	
    phoneno int(12) NOT NULL
    /*......*/
);
//class my data php

<?php

include('connect.php');
class InsertMydata {
    public function insertnow($fname, $phoneno) {
        $connect = new Connect;
        $insrt = $db -> prepare('INSERT INTO mydata (fname, phoneno) VALUES (?,?)');
		$insrt -> execute(array($fname, $phoneno));		
	}
}
?>


//insernow validate form

<?php
include('../classs/mydata.php');
//Declare data and error arrays
$errors = [];
$mydara = [];
if(!preg_match('/^[a-zA-Z]{4,15}$/', $_POST['fname'])) {
	$errors['fname'] = 'Enter full name!';
}
//this block not working even the phone exist
$connect = new Connect;
$phoneno = $_POST['phoneno'];
$checkiexist = $connect -> prepare('SELECT * FROM mydata WHERE phoneno = ?');
$checkiexist -> execute([$phoneno]);
if($checkiexist->rowCount() > 0) {
      $errors['phonenoexist'] = 'Try another phone number!';
 }
if(!empty($errors)){
      $data['success'] = false;
      $data['errors'] = $errors;
}else{
      $data['success'] = true;
      $data['message'] = 'success message!';  
      $mydata = new InsertMydata;
      $mydata -> insertnow($fname, $phoneno);
}
echo json_encode($data);

?>

//my ajax

$("#insertbtn").click( function(e) {
     var fname = $('#fname').val(),
         phoneno = $('#phoneno').val(),
           
       $.ajax({
           url: 'insertnow.php',
           type: 'POST',
           data: {fname:fname, phoneno:phoneno},
           dataType: "JSON",
           encode: true,
        }).done( function (data) {
         
         if (data.success == false) {
           if (data.errors.fname) {
                 $('#fname').append('<p class="text-danger">' + data.errors.fname + '</p>');
            } 
            if (data.errors.phonenoexist) {
              $('.card-header').append('<div class="alert alert-info alert-dismissible" role="alert">
                <button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>'+data.errors.phonenoexist+'</div>');
            }
            } else {
             $('.card-header').append('<div class="alert alert-success alert-dismissible" role="alert"><button type="button" class="close" data-dismiss="alert" aria-label="Close"><span aria-hidden="true">×</span></button>'+data.message+'</div>');
        }
    });
       e.preventDefault();
    });


//the problem is, the code insert data even if the phone exist why?

the problem is, the code insert data even if the phone exist why?