<?php
if(isset($_POST['create_accnt'])){
    // Verify Information first.
    if(strlen($_POST['fname']) === 0 || strlen($_POST['lname']) === 0 || strlen($_POST['mail']) === 0){
        die('Not all Fields were entered properly. Go <a href="Javascript: history.go(-1)">Back</a> and correct missing fields.');
    }
    // POST Variables are stored in the array $_POST, not _$POST
    // You will also want to look into 'SQL Injection protection' for the future.
    $lname = $_POST['lname'];
    $fname = $_POST['fname'];
    $mail = $_POST['mail'];
    mysql_connect("localhost", "root", "") or die("Unable to connect to MySQL");

    //echo "Connected!!<br>";// You have an error message that tells you if you are not connected, so no need for the connection varification. A lack of error message is confirmation enough.

    mysql_select_db("logindb") or die ("No such Database Found");

    //checking if data is already available or not
    $query = mysql_query("SELECT fname,lname FROM login_details WHERE fname='{$fname}' AND lname='{$lname}' AND mail='{$mail}'"); // You had a typo lame -> lname

    //tells the no of rows for which the data matched
    $rows = mysql_num_rows($query);

    //if data already present
    if($rows > 0){
        echo "data already present fill again";
    }else{//if data is not in database
        echo "value inserted";
    }
}else{
    echo "not connected!!";
}
?>