Bootstrap form submission with PHP showing error

i have used the bootstrap inline form. Here's the code:

<form class="form-inline" action="php/signup.php" method="post">
                <div class="form-group">
                    <label class="sr-only" for="firstname">First name</label>
                    <input type="text" required class="form-control" id="exampleInputEmail2" name="firstname" placeholder="First name">
                </div>
                <div class="form-group">
                    <label class="sr-only" for="middlename">Middle name</label>
                    <input type="text" class="form-control" id="exampleInputEmail2" name="middlename" placeholder="Middle name">
                </div>
                <div class="form-group">
                    <label class="sr-only" for="lastname">Last name</label>
                    <input type="text" required class="form-control" id="exampleInputPassword2" name="lastname" placeholder="Last name">
                </div>


                <br/>
                <br/>

                <div class="form-group">
                    <label class="sr-only" for="username">User name</label>
                    <input type="text" required class="form-control" id="exampleInputPassword2" name="username" placeholder="Desired user name">
                </div>

                <br/>
                <br/>

                <div class="form-group">
                    <label class="sr-only" for="password">Password</label>
                    <input type="password" required class="form-control" id="exampleInputPassword2" name="password" placeholder="Password">
                </div>

                <br/>
                <br/>

                <div class="form-group">
                    <label class="sr-only" for="repassword">Re enter password</label>
                    <input type="password" required class="form-control" id="exampleInputPassword2" name="repassword" placeholder="Re enter password">
                </div>

                <br/>
                <br/>

                <div class="form-group">

                    <input type="email" required class="form-control" id="exampleInputPassword2" name="email" placeholder="E-mail address">
                </div>

                <br/>
                <br/>

                <button type="submit" name="btnSubmit"> Sign up</button>
        </form>

I also a PHP backend for submitting the form data:

<?php
session_start();
include("connect_to_mysql.php");
if(isset($_POST("btnSubmit")))
{
    $firstname = mysql_real_escape_string($_POST("firstname"));
    $middlename = mysql_real_escape_string($_POST("middlename"));
    $lastname = mysql_real_escape_string($_POST("lastname"));
    $username = mysql_real_escape_string("username");
    $password = mysql_real_escape_string("password");
    // check the re entered password using jquery on the client machine itself
    $email = mysql_real_escape_string("email");


    /*firstname varchar(255),
                middlename varchar(255) NOT NULL,
                lastname varchar(255) NOT NULL,*/
    $query = "create table if not exists authenlist(
                id int NOT NULL AUTO_INCREMENT,
                username varchar(255) NOT NULL,
                password varchar(255) NOT NULL,
                PRIMARY KEY (id))";

    $i = mysql_query($query) or die("authentication table not created");

    if($i)
    {
        $query = "select * from authenlist where username='$username'";

        $o = mysql_query($query) or die("query not executed");

        if($o)
        {
            if(mysql_num_rows($o) == 0)
            {
                $query = "insert into authenlist values('', '$username', '$password')";
                $_SESSION['username']=$username;//to register user
                header("location:home.php");
                exit();
            }
            else
            {
                header("status: 404 not found");
            }
        }
    }
}
?>

When i try to submit the form there is always a fatal error: Fatal error: Can't use function return value in write context in C:\wamp\www\the_unknown\php\signup.php on line 4.

I am unable to understand what is the error in the process of form submission. I am in immediate need of this form and I really need it to work. Please can somebody help me out with the problem I am facing. Thanks in advance.