Convert static navigation bar to dynamic - php

Question

From the research I have done, I think it has to do something with the include() function linking to the nav.php file. What I am trying to accomplish is taking an older huge site with every element being static ones and converting it to have dynamic elements so that the updating is faster. The first step was the nav bar and I can't seem to get it to work.
The route to the nav bar is inputs/nav.php, and I'm trying to use include() to put the nav in:
<?php include("inputs/nav.php"); ?>
It works for the files at the same level as the inputs folder, but if a page is at any other level of files, it does not. Anyone got any ideas of what is going on and how to fix?
Thanks in advance!
<header>
<div id="mastHead">
<div id="fss_logo">
<a href="index.html">
<img src="images_folder/logos/35fss_logo4.png" alt="35 FSS" />
</a>
</div>
<!--End FSS logo-->
<div id="facility_title">
<div id="rotating-item-wrapper_main">
<a href="facebook_links.html" onClick="return popup(this, 'facebook')">
<img id="facebook_logo6" src="images_folder/logos/facebook_circle2.png" alt="Facebook Logo" class="rotating-item" />
</a>
<a href="http://ice.disa.mil/index.cfm?fa=site&site_id=7&dep=DoD" target="_blank">
<img id="ice_logo2" src="images_folder/logos/ice.png" alt="ICE Logo" class="rotating-item" />
</a>
</div>
<!-- END: Rotating images images -->
</div>
<!--End facility_title-->
</div>
<!--End Mast Head-->
<!--------------------------- NAV INCLUDE ----------------------------->
<?php include( "inputs/nav.php"); ?>
<!--------------------------- NAV INCLUDE ----------------------------->
<!-- /nav-wrap -->
</div>
</header>