Joontje48 on "PHP file_exists function won't work"

I have a WordPress website (still running locally) with different pages. I want each page to display its own unique header image (using the page name as image name), and am able to get that part working smoothly. But, when no header image is available (yet) for that specific page, I want to display a placeholder image. To do this, I use the file_exists function. It doesn't work, however. I use this code (note: this is a simplified/altered version. I don't bother you with the full code since I don't even get this simple code to work).

<img src="<?php
if (file_exists('/images/headers/home.jpg')) {
    echo '/wp-content/themes/theme/images/headers/home.jpg';
}
else {
    echo '/wp-content/themes/theme/images/headers/placeholder.jpg';
}
?>" />

The problem is: even though home.jpg exists, it keeps showing me the placeholder.jpg.

I already tried following file paths:
- Full link
- /wp-content/themes/theme/images/headers/home.jpg
- /images/headers/home.jpg
- home.jpg