Php include header not showing on some machines - php

Question

So i have an header and footer in seperate files, i include them via the php include method. On my machine it works on chrome and edge. When i go to a other pc it works on chrome but not on edge. Now when it doesn't load it actually loads but dissapears directly after. It is only on screen for a short flash.
My file structure is as following:
2.10.0
index.php
assets
headernav.php
In my index.php file i use this code to include.
<?php include("assets/headernav.php"); ?>
The headernav.php file it includes is as follows
<script>
function openmen() {
document.getElementById("menu").style.marginLeft = "0";
document.getElementById('menubtn').setAttribute("onClick", "closemen()");
document.getElementById("content").style.paddingLeft = "250px";
document.getElementById("searchbox").style.left = "240px";
}
function closemen() {
document.getElementById("menu").style.marginLeft = "-230px";
document.getElementById('menubtn').setAttribute("onClick", "openmen()");
document.getElementById("content").style.paddingLeft = "20px";
document.getElementById("searchbox").style.left = "10px";
}
function opensettings() {
document.getElementById("settings").style.display = "block";
}
function closesettings() {
document.getElementById("settings").style.display = "none";
}
</script>
<settings_background id="settings">
<settings>
<h2>Instellingen</h2><i id="closebtn" onclick="closesettings()" class="fas fa-times"></i>
<a>Eenheid</a>
<form method="POST" action="includes/units.inc.php">
<input placeholder="Naam" class="inputmaterial" type="text" name="naam" value="<?php echo $_SESSION['naam']; ?>">
<input placeholder="Callsign" class="inputmaterial" type="text" name="callsign" value="<?php echo $_SESSION['callsign']; ?>">
<select Placeholder="department" class="inputmaterial" name="department">
<option value="<?php echo $_SESSION['department']; ?>"><?php echo $_SESSION['department']; ?></option>
<option value="Police">Police</option>
<option value="Traffic police">Traffic police</option>
<option value="Fire">Fire</option>
<option value="Ems">Ems</option>
<option value="Milatary police">Milatary police</option>
<option value="Siu">Siu</option>
</select>
<input placeholder="rang/functie" class="inputmaterial" type="text" name="rang" value="<?php echo $_SESSION['rang']; ?>">
<input placeholder="Naam" type="hidden" name="id" value="<?php echo $_SESSION["id"]; ?>">
<input type="hidden" name="return" value="index.php">
<br><br><br><input style="float:left;"class="button_grey" type="submit" name="cookie" value="Save">
</form>
<a>Account</a>
<form method="POST" action="login_v3/account.inc.php">
<input placeholder="Huidig wachtwoord" class="inputmaterial" type="password" name="pwdold">
<input placeholder="Nieuw wachtwoord" class="inputmaterial" type="password" name="password">
<input placeholder="uid" type="hidden" name="uid" value="<?php echo $_SESSION['id']; ?>">
<br><br><br><input class="button_grey" type="submit" name="updatepassword" value="Save">
</form>
<a>Personalisatie</a>
<form action="includes/units.inc.php" method="POST">
<input type="checkbox" id="id-name--1" name="meldingen" class="switch-input" <?php if(isset($_SESSION['notificaties'])){echo "checked";} ?>>
<label for="id-name--1" class="switch-label">Meldingen <span class="toggle--on">Aan</span><span class="toggle--off">Uit</span></label>
<input type="checkbox" id="id-name--2" name="darkmode" class="switch-input" <?php if(isset($_SESSION['darkmode'])){echo "checked";} ?>>
<label for="id-name--2" class="switch-label">Darkmode <span class="toggle--on">Aan</span><span class="toggle--off">Uit</span></label><br><br><br>*darkmode is nog in beta
<input placeholder="Naam" type="hidden" name="id" value="<?php echo $_SESSION["id"]; ?>">
<br><br><input type="submit" name="personalisatie" value="Save" class="button_grey">
</form>
</settings>
</settings_background>
<nav_menu id="menu"><div class="section">
<i class="fas fa-phone"></i> Meldingen
<i class="fas fa-id-card"></i> Database
<i class="fas fa-eye"></i> Gezocht
<i class="fas fa-map-marked-alt"></i> Kaarten
</div>
<div class="section">
<p>Verslagen</p>
</i> Verslagen
<a><i class="fas fa-notes-medical"></i> Nieuw verslag</a>
</div>
<div class="section">
<p>Registratie</p>
<i class="fas fa-bell"></i> Maak melding
<i class="fas fa-receipt"></i> Bekeuring
<i class="fas fa-exclamation"></i> Waarschuwing
<i class="fas fa-mask"></i> Misdrijf
<a><i class="fas fa-eye"></i> Arrestatiebevel</a>
</div>
<div class="section">
<p>Database toevoeging</p>
<a><i class="fas fa-car"></i> Voertuig</a>
</div>
<div class="section">
<p>Meldkamer</p>
<i class="fas fa-headset"></i> Meldkamer
</div>
<p>
© www.lscdb.nl
</p>
</nav_menu>
<nav_bar><div class="nav_bar_left"><i id="menubtn" onclick="closemen()" class="fas fa-bars"></i><img src="../img/lscdb_logo_white.png"></div><div class="logged_in"><?php echo $_SESSION['naam']; ?> | <?php echo $_SESSION['callsign']; ?></div><i class="fas fa-question"></i><i class="fas fa-sign-out-alt"></i><i onclick="opensettings()" class="fas fa-cog"></i></nav_bar>
So to conclude, my headernav file i include using php works on some machines, while on others loads and dissapears directly after.

Answer 1

So i found a solution.... finally.
It turns out the header that has a position:fixed allways needs top:0; and left:0;, even if the header is allready in the correct position.