Insert data into database

<?php

require "connection.php";


if ($_POST['firstname'] != "" && $_POST['lastname'] !="" && $_POST['email'] != "" && $_POST['cemail'] !="" && $_POST['password'] !="" && $_POST['birthday'] !="" && $_POST['gender'] !="")
{
	$firstname = $_POST['firstname'];
	$lastname = $_POST['lastname'];
	$email = $_POST['email'];
	$cemail = $_POST['cemail'];
	$password = $_POST['password'];
	$birthday = $_POST['birthday'];
	$gender = $_POST['gender'];

	
	if($email != $cemail)
	{
		echo "email not same";
		die;
	}
	
	$query1="SELECT * FROM users WHERE firstname = '$firstname' ";	
	
	
	
	$result = mysqli_query($conn,$query1)
	or die(mysqli_error($conn));  

	
	if(mysqli_num_rows($result) != 0)
	{
		header("Location: index.php/?a=1");
	}
	else {
	$query = "INSERT INTO users (firstname,lastname,email,password,date of birth,gender) VALUES ('$firstname','$lastname','$email','$password', '$birthday' , '$gender')";
	$data = mysqli_query($conn,$query)or die(mysqli_error($conn));
	
	header("Location: index.php/?a=2");
	  
	}
	
}

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I have this code to insert what was in my form to here, the data passes but it just doesnt add to my database, anyone know why?