Nothing happen when i click on submit button in a form

I'm trying to check if the input value is existe in the database or not

but, when i click on submit button nothing happen(executed)

here's a part of html code

<div id="main">
	<div style='width:900px!important;padding-left:30px!important;' id="content">
					<h4>Via deze pagina kunt u rechtstreeks inloggen op uw e-mailbox.</br></br>
Wat is je e-mailadres?</h4>
<div style="clear:both;height:0px;"></div>	
<div class="s01">
      <form  action="checker.php" method="post" >
        <div class="inner-form">
          <div class="input-field first-wrap">
            <input id="username" type="text" name="username" placeholder="Username" />
		  <div><p style="color:white;margin-right: 30px;font-size: 20px;">@</p></div>
          <div class="input-field second-wrap">
            <input id="location" type="text" placeholder="Domain name" />
          <div class="input-field third-wrap">
            <button class="btn-search" name="submit" type="button">Search</button>
	<div><h4>Stelt u liever de mailaccount in op uw eigen systeem via Microsoft Outlook? Neem dan contact met ons op voor gratis ondersteuning.</h4></div>

and here's the php code in a file next to the html file named checker.php

$username = $_POST['username'];   
$mysqli = new mysqli("localhost","myUsername","myPassword","mydatabaseName");

// Check connection
if ($mysqli -> connect_errno) {
  echo "Failed to connect to MySQL: " . $mysqli -> connect_error;

    //check if username exists
    $query = "SELECT * FROM wordpress_server WHERE user_emails = '".$username."'";
    $result = mysqli_query($mysqli,$query);

            header("Location:"); //just for test

                    header("Location:"); //just for test