<form>
<input id="search" type="text" size="30" onkeyup="showresult(this.value)" >
<div id="suggest"></div>
</form>


<script>
function showresult(val){
    if(val.trim() == ""){

    }else{
        var xttp = new XMLHttpRequest() ;
        xttp.onreadystatechange = function () {
            try {
                var s = xttp.responseText ;
            }
            catch (e){
                window.alert(e.message) ;
            }

            if(s.match("zerorow")){
                document.getElementById("suggest").innerHTML = "zero" ;
            }else {
                try {
                    window.alert(s) ;
                    var arr = JSON.parse(s) ;

                }
                catch(err) {
                    document.getElementById("suggest").innerHTML =  err.message;
                }



            }
        };
        xttp.open("POST" , "tst.php" ,true) ;
        xttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded") ;
        xttp.send("val="+val) ;
    }
}

<?php
$servername = "localhost";
$dbusername = "mamad";
$dbpassword = "";
$dbname = "t1" ;

// Create connection
$conn = mysqli_connect($servername, $dbusername, $dbpassword , $dbname);

if (!$conn) {
echo die("Connection failedddddddddd: " . mysqli_connect_error());

}else{

$val = $_POST["val"] ;
$sql = "SELECT tag FROM tags WHERE tag LIKE '$val%'";
$result = mysqli_query($conn, $sql);

if($result){
    if(mysqli_num_rows($result) > 0){
        $arr = mysqli_fetch_all($result , MYSQLI_NUM) ;
        echo json_encode($arr) ;
    }else{
        echo "zerorow" ;
    }
}else{
    echo die("faileddd " . mysqli_connect_error());
}

}
?>

$arr = mysqli_fetch_all($result , MYSQLI_NUM) ;
echo json_encode($arr) ;

var arr = JSON.parse(s) ;