php copy and file存在功能：我可以使用绝对位置

I am very new to php. I have a pdf file at the absolute location "/home/abc/def.pdf". I have a php code (displaypdf.php) in /var/www/html which looks like

<?php
 $pdfloc = $_GET['pdfl'];
 if (file_exists($pdfloc)){
     if (!copy($pdfloc, '/tmp/temp.pdf')) {
         echo " destination file not found";
     }
     else {
         echo " file copied";
     }
 }
 else {
     echo " source file not found";
 }
?>

when I do http://localhost/displaypdf.php?pdfl="/home/abc/def.pdf" it says "source file not found" but I do have the source file with -rwxr-xr-x permission. What am I doing wrong?