What prevents me from opening a text file in php using fopen()

I have written this chunk which always fails to open the file. I cannot figure out why

<?php
$file=‘/var/www/html/messages.txt’;
$fp=fopen($file, ‘a+’) or die(“fopen failed);
?>

The code always spits out “fopen failed” and I can’t figure out why. I’m hoping someone can help.